How much sleep does a Warfighter need?
What's the best way to stay rested when sleep is difficult or irregular?
From the Field
What's the best way to stay rested when sleep is difficult or irregular?
An overview of how Warfighters can improve their sleep
Most individuals need seven to eight hours of night sleep to function optimally for maximum alertness. Because deployed Warfighters rarely get eight hours of uninterrupted sleep, they should aim to improve their sleep by adding power naps. However, eight hours of naps do not equal eight hours of continuous sleep. In a deployed environment, sleep quality is likely compromised, so behavioral methods (described below) for making your sleep optimal should be used.
Myths and/or Claims
Probably the most pervasive myth around sleep is that it’s possible to function well over a long period of time with just a few hours of sleep. People also often think that by sleeping ahead in anticipation of missed sleep, hours can be added to your sleep “bank,” and that anytime is as good as another to catch up by napping. Or, conversely, they think it’s better to stay awake rather than catch a few minutes of sleep, if that’s all that can be done. Also, people sometimes ride the high of having been awake a full day, and think they can coast on that high – sleep isn’t necessary. Another myth is that physical activity makes you sleepy.
The fact is, most people need seven to eight hours of sleep for maximum sustained alertness and cognitive functioning. However, many people don’t get that amount. The body cannot be trained to need less sleep than the normal baseline amount. Though, genetic differences in sleep needs exist, not even one “super-hero” type person can get by on no sleep at all, and very few can function with just a few hours over time. Many individuals can function well on little sleep for short durations (i.e. one night of restricted sleep) – particularly with caffeine or other stimulants – but some cannot. However, limited sleep over the long-term will impair functioning. Sleep is not dependent on individual characteristics – all individuals who sleep only four to five hours of sleep each night will experience some loss of performance.
Maximum sleepiness is between 0300 to 0500 and 1300 to 1500. If you miss sleep and need to make it up, sleeping during these key hours is optimal as your body is physiologically more prepared for sleep. Also, performing with maximal alertness is reduced between midnight and 0800. Experts are clear: any sleep is better than no sleep, especially after having been awake for an extended period of time. Choosing to stay awake, rather than catching a few minutes of sleep when the opportunity presents itself, is riding a high that can be detrimental. After being awake for 18 hours, "mental and motor skills deteriorate as much as they do when people are drunk on alcohol.” With total sleep deprivation, performance typically declines by 25% every 24 hours (depending on the type of performance) being measured.11
Finally, exercise is one of the most effective ways of combating sleepiness. Even stretching and isometric exercises can help.
Getting less than four hours of sleep can make you prone to moments of uncontrolled sleep attacks and accidents. Sleep loss influences mental performance through decreasing the ability to make rational decisions, accurately assess risk, correctly process information and ensure coordinated motor skills – all important aspects of mission success. Additionally, sleep loss can make people more irritable, less happy, worsen overall mood, and cause physical effects, like gastrointestinal problems.
Summary for Military Relevance
Operational Commanders should weigh the sleep needs of Warfighters against mission needs. Without pharmaceutical assistance, one should:
- Optimize sleep quality through blocking out light (use of sleep masks) and noise (use of earplugs when appropriate), if receiving less than eight hours of sleep.
- Maintain separate night and day sleep tents to reduce light, sleep disruptions, and improve sleep quality.
- Decrease caffeine intake at least three to five hours before sleep.
- Use 40-minute naps placed strategically to reduce total hours of wakefulness.
- Schedule time for Warfighters to decrease their sleep debt (i.e. sleep loss incurred) by sleeping longer during the night to reset, after returning from a mission.
- Though research indicates that most adults need seven to eight hours of sleep (with some needing six hours and others up to 10 hours) for optimal cognitive alertness [1,2], most Warfighters will find this difficult to obtain when deployed. Sleep habits can be optimized for maximum performance.
- Criteria for Operational Commanders: Weigh the needs of sleep against the mission. Sleep can be optimized without pharmaceutical assistance by:
- Blocking out light (use of sleep masks) and noise (use of earplugs, unless from an operational point of view it's impossible);
- Decreasing use of caffeine at least three to five hours before sleep;
- Strategically placing 40 minute naps to reduce total hours of wakefulness;
- Scheduling time upon return from a mission for Warfighters to reset and decrease their sleep debt (sleep loss incurred) by sleeping longer during the night;
- Maintaining separate night and day sleep tents to reduce light, sleep disruptions, and improve sleep quality.
- Quality of sleep and time of day (best at night) for sleeping are key factors that contribute to feeling rested and alert.
- Cognitive performance will continue to decline over the course of a few days according to the total hours of wakefulness.
- Use naps strategically to decrease the total awake hours (naps are best between 0300 to 0500 and from 1300 to 1500).
- Individuals can function on little sleep for short periods of time (i.e. one night of restricted sleep) - particularly with caffeine or other stimulants. Limited sleep (significantly less than 8 hours) over the long-term (i.e. weeks to months) can impair functioning and increase the risk of making errors.
- Any sleep is better than no sleep.
How much sleep an individual should receive for optimal performance is a popular question with many pervasive myths. Adequate sleep is necessary for optimal cognitive performance yet Warfighters often find obtaining adequate sleep difficult in operational settings. The following sections will discuss the myths surrounding sleep, the facts of sleep management, how sleep impacts performance, and provide strategies for how to optimize sleep quality operationally for maximum alertness.
Myths and/or Claims
Probably the most pervasive myth around sleep is that it is possible to function well over a long period of time with just a few hours of sleep. People also often think that by sleeping ahead in anticipation of missed sleep, hours can be added to your sleep “bank,” and that anytime is as good as another to catch up by napping. Or, conversely, that it is better to stay awake rather than catch a few minutes of sleep, if that’s all that can be done. Also, people sometimes ride the high of having been awake a full day, and think they can coast on that high – sleep isn’t necessary. Another myth is that physical activity makes you sleepy.
How much sleep does an individual need?
The number of hours each individual needs can vary, however, the general guidelines are that most adults need eight hours of sleep [2,3], with some needing six hours and others up to 10 hours to optimally function during wakefulness . Optimal sleep is critical for maximum alertness with easy and automatic functioning . If eight hours of uninterrupted sleep cannot be obtained, then add naps to get a sum total of eight hours of cumulative sleep  (Note: eight hours of naps do not equal eight hours of continuous sleep). If one did not sleep a full night, then a power nap and eight hours night sleep the next day will restore the individual; the number of hours lost does not necessarily have to be regained. Remember, there is no adaptation to sleep deprivation.
- Keep in mind that after about 20 hours of wakefulness, cognitive and coordination abilities decline similar to the effect of alcohol .
- With total sleep deprivation, performance typically declines by 25% every 24 hours (depending on the type of performance) being measured .
- Sleeping less than four hours can make you prone to moments of uncontrolled sleep attacks (i.e. falling asleep) and various kinds of accidents .
What are the effects of sleep loss?
Sleep loss can cause cognitive (i.e. decreases in alertness, reaction time, ability to remember, processing speed, decision making and motor skills), emotional (i.e. increases in negative emotions and overall worsened mood) [1,4], and physical (i.e. gastrointestinal complaints) effects. Even losing one to two hours of sleep than usually required decreases cognitive performance and alertness significantly . The effects of loss of sleep on psychomotor vigilance skills with nine hours, seven hours, five hours and three hours of sleep over a seven-day period on inactive individuals have been clearly demonstrated in a lab environment . Loss of sleep severely impacts cognitive performance – the more sleep loss, the greater the degradation; only three hours of sleep can severely degrade performance levels.
Although sleep deprivation is sometimes a way of life for commanders and warfighters, all individuals are affected by sleep loss and cannot adjust to less sleep, regardless of the circumstances. However, individuals vary markedly in their performance responses to sleep loss .
Signs and Symptoms of Sleep Loss (aka fatigue):
Sleep loss can cause forgetfulness, fixations, poor decision making, slower reaction time, lethargy, apathy, reduced vigilance, bad mood, poor communication and nodding off [1,4].
Individuals who obtain less sleep than they usually need each night create a sleep debt. For example, if eight hours of sleep is needed for optimal functioning and only six hours are obtained for two nights – then the sleep debt is four hours. Sleep debt is like a bank – if you withdraw four hours from it, it should be paid back the next night if possible, or the sleep debt grows bigger [4,7]. However, less sleep is required to repay the sleep debt than the actual debt . Individuals can actually recover from sleep deprivation without paying off the entire sleep debt. In the above example, if four hours of sleep were withdrawn (two hours for each night), then one power nap each day and a full night sleep during the third night should restore performance.
When to sleep is dictated by circadian rhythms regulated by the brain on a 24-hour basis [1,3,8]. Circadian rhythms are linked to core body temperature, so optimal sleep hours are during the night when core body temperature is lowest between 0300 and 0500 - when sleepiness is greatest and performance abilities are lowest . Also, the circadian schedule is on local time because the environmental cues, such as sun and social patterns, drive the rhythm. When crossing time zones, the circadian clock needs time to adjust – from several days to several weeks . Factors that influence this adaptation are how many time zones are crossed, and whether the flying is eastbound or westbound; taking an eastward flight takes longer to acclimate than a westbound flight .
Optimal Sleep Times:
Maximum sleepiness is 0300 to 0500 and from 1300 to 1500 . If sleep is missed and needs to be made up, sleeping in these key hours is optimal. Vigilance is typically reduced from midnight to 0800 due to circadian rhythms.
Extensive research has documented sound operational strategies for optimizing performance and alertness through sleep [1,4]. Sleep requirements and the effects of sleep deprivation are quite variable and Commanders should act properly if they notice any symptoms of sleep deprivation. Four crucial operational issues should be considered for sleep management .
Operational Issue #1:
Most humans physiologically require seven to eight hours of sleep a night for optimal performance and alertness. Since it is not dependent on individual characteristics, all individuals with significantly less sleep than they usually require will experience some degradation of performance; not even one “super-hero” type person can get by on no sleep at all [1,10] and very few can function well on minimal sleep. Although it is impossible to predict how sleep loss will affect an individual’s performance , forecasting techniques for predicting performance decrements of an individual during operations are being developed . Some individuals may function well with 6-7 hours and some may need more than 8 hours of sleep for maximum alertness. Likewise, performance decrements may be great in some and minimal in others after extended work periods .
Operational Issue #2:
The length of the continuous wakefulness. Research comparing shift lengths indicates that shifts greater than 12 hours are associated with significant decrements in cognitive performance and increases in errors and injuries and accidents and injuries increase threefold after 16 hours of continuous work [2,3].
Operational Issue #3:
Time of day. Circadian rhythms dictate human performance and alertness through night flying, time zone changes and day/night duty shifts [4,8].
- The circadian trough (0300 to 0500) and, in general, nighttime (midnight to 0600) are critical periods for decrements in performance and alertness. Therefore, working a 14-hour duty period during the night is not the same as working it during the day.
- Time zone changes disrupt internal circadian rhythms.
Operational Issue #4:
Minimize the cumulative effects of loss of sleep. The longer one is without sleep, the greater the degradation in performance. If less than seven to eight hours per 24 hour period of sleep are obtained, then approximately two nights of recovery sleep are usually needed to resume baseline levels for optimal performance (more might be needed, depending on the amount of the sleep debt) [3,7,8].
Summary for Military Relevance
Fatigue Countermeasures are preventive strategies used before duty time and during rest periods, whereas operational countermeasures are those used on the job to optimize performance. Countermeasures for managing fatigue should consider individual vulnerability or resilience to the consequences of sustained efforts with limited sleep, as one size fits all strategies are not optimal .
Preventive strategies, aimed at the physiologic aspects of fatigue caused by sleep loss [1,4], include:
Minimizing sleep loss: Do not begin a new schedule with an existing sleep debt – this “requires two nights of unrestricted sleep.”
- Emerging research suggests that it might be possible to bank sleep ahead of time prior to sleep loss. Banking sleep prior to sleep loss may mitigate some of the performance decrements caused by sleep loss, help with alertness and speed recovery - all important factors for operational environments .
- If it takes more than 15-30 minutes to fall asleep, get out of bed and undertake a relaxing activity until sleepiness occurs.
- Short naps of only 40 minutes are useful if sleepy prior to work, and minimize the chance of entering a deep sleep and being groggy upon waking.
- Any sleep is better than no sleep [9,10].
- Maintaining good sleep habits: Having a regular pre-sleep routine is a bodily cue to prepare for sleep. Try meditation, yoga and progressive muscle relaxation before going to sleep.
- Bedrooms should be associated with relaxing memories, not work or stress and should be dark and quiet – use earplugs or white noise machines to block out noise – and cool, as cooler environments are best for optimal sleep (if possible).
- Eat a light snack or a small drink, if hungry or thirsty before sleep.
- Reduce caffeine intake prior to bedtime. Caffeine can remain active for three to five hours, and sometimes up to 10 hours; it can lead to lighter sleep with reduced total sleep time.
- Avoid strenuous exercise for six hours prior to sleep (due to the physiologic activation that occurs). However, regular exercise is “physically restorative.” For non-sleep deprived individuals, moderate exercise also improves cognitive performance immediately afterward for 20-30 minutes, however, be careful doing strenuous exercise as it may create more fatigue .
Operational Countermeasures include:
- Social interaction: Talking and being actively engaged in a conversation can maintain alertness.
- Posture: It is easier to stay awake when standing or sitting rather than lying down – walking around is best .
- Physical activity: Exercise is a very effective method for combating sleepiness . Stretching and isometric exercises can help too .
- Distractions: Writing or chewing gum can help individuals stay awake.
- Caffeine: Caffeine can be used strategically to stay awake. Taking 200 mg every two hours can rescue performance for 74 hours of wakefulness . However, taking it three to four times per day should be enough; more caffeine can cause problems.
- Diet: Carbohydrates can increase alertness initially but may be followed by a drop in blood glucose and a decrease in alertness. Consider consuming meals balanced with high fiber carbohydrates, lean proteins, and healthy fats.
- Strategic napping: Cockpit crew members allowed to nap for 40 minutes during four nine-hour transpacific flights were found to perform better and be more alert during the last 90 minutes of the flight than a comparison group. Napping is important even with multi-day sleep loss – after almost 2 days of sleep deprivation, a two-hour nap can maintain subsequent performance at 70 percent of baseline .
- Timing: Taking naps during the circadian trough or between 0300 to 0500 and 1300 to 1500 helps as that is when the body is naturally ready to sleep. The lowest levels of functioning within this trough are from 0300 to 0500. Note, it is easier to sleep between 1300 to 1500 and most difficult to sleep between 1700 and 2100 [1,4].
- Scheduling: Schedule naps as close as possible to beginning a long duty shift; time awake since last sleep is an important determinant of alertness and cognitive performance .
- Grogginess (i.e. sleep inertia): individuals waking from a nap as well as from a regular night sleep are usually groggy for 30 minutes after waking up. Bright light and caffeine may help individuals wake up faster.
Sleep Guidelines for Military Operations:
Sleep deprivation causes greater mental consequences than physical consequences. Therefore, the following strategies are presented to mitigate those consequences.
Most individuals need seven to eight hours of night sleep to function optimally for maximum alertness. Because deployed Warfighters rarely are able to obtain eight hours of uninterrupted sleep, they should aim to improve their sleep quality and add power naps, if possible. However, eight hours of naps do not equal eight hours of continuous sleep. In a deployed environment, sleep quality is likely compromised so the behavioral methods described above for enhancing sleep quality should be used for optimizing performance.
References and Resources
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- Rosekind MR, Boyd JN, Gregory KB, Glotzbach SF, et al. Alertness management in 24/7 settings: lessons from aviation. Occup Med. 2002;17(2):247-59, iv. PMID: 11872439
- Caldwell JA, Mallis MM, Caldwell JL, Paul MA, et al. Fatigue countermeasures in aviation. Aviat Space Environ Med. 2009;80(1):29-59. PMID: 19180856
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- Belenky, G., Wesensten, NJ, Thorne, DR, Thomas ML. et al., Patterns of performance degradation and restoration during sleep resriction and subsequent recovery: a sleep dose-response study. J Sleep Res. 2003;12(1):1-12.
- Rupp, T., Wesensten, N., Bliese, P. & Balkin, T. (2009). Banking sleep: Realization of benefits during subsequent sleep restriction and recovery. Sleep, 32(3), 311-321.