Sleep deprivation means receiving less sleep than a person needs, up to the complete absence of sleep during an extended period of wakefulness. In military isolation, this loss of sleep particularly affects sustained attention, the acquisition of new information and reliable decisions. It can result from movement and observation tasks, a lack of sleep opportunities or enforced wakefulness in captivity. Physical exhaustion may accompany it but is a separate burden.
Sleep loss and physical exhaustion
A long march carrying a load demands muscular effort and physical energy. Putting down the load and stopping work reduces that demand. Sleep loss can also develop while someone remains almost motionless watching an approach. A break from observation or lying quietly can reduce the fatigue caused by the continuous task; lost sleep is made up through actual sleep. Both may be needed at the same time: less physical work and an opportunity to sleep.
Sleep may be missed completely on one occasion or repeatedly cut short over several rest periods. Repeated awakening also reduces its duration and interrupts its continuity. The time someone spends lying concealed therefore says little about how much they have actually slept. Lying with eyes open, waiting for sounds and startling at every movement are periods of wakefulness.
Sleep pressure and circadian rhythm
The biological pressure to sleep increases with time spent awake. This part of sleep regulation is called homeostatic sleep pressure. The internal daily rhythm, or circadian rhythm, also influences when alertness is easier to maintain and when sleepiness increases. The two act together. Previous sleep loss and further wakefulness burden attention even when the circadian rhythm temporarily favours feeling more alert.
For an evading person, a temporary revival does not show that the sleep deficit has been corrected. Night movement intended to reduce visibility can also fall during a period of pronounced sleepiness. This combines two different risks: detection in daylight and errors caused by poor alertness. The movement decision therefore also needs to account for the next reachable opportunity to sleep.
Attention lapses and microsleep
Sustained observation requires acquiring cues over time rather than only looking occasionally. Sleep deprivation slows responses and increases lapses. A person may identify one cue correctly and shortly afterwards miss another entirely. While watching a track, such a gap may be enough to miss someone approaching.
Microsleep is a brief involuntary episode of sleep during intended wakefulness. The person may not notice it. An attention error alone does not establish that microsleep occurred; both matter in practice when an approach is being watched, a vehicle driven or a hazardous section of terrain crossed. Being able to speak or walk says little about whether new information is being acquired continuously.
Warning signs include repeatedly nodding off, losing track of the task currently being performed or gaps in the memory of the most recent stretch of a journey. Someone who cannot place the last landmark seen, or has to start the same operating step repeatedly, has a concrete reason to interrupt the action. The judgement “I am still alert enough” also becomes unreliable under sleep loss. Errors therefore need attention before the person feels completely exhausted.
Memory and a changing situation
Sleep deprivation can impair the acquisition and processing of new information. Information already acquired may still be available briefly while new cues are incorporated less successfully. Someone may therefore describe the planned route correctly yet fail to account adequately for a checkpoint just observed or a new movement of people. Adjustment to changed feedback can also suffer: noticing an error may not reliably produce a change in the next action.
In navigation, this affects questions such as whether a terrain feature actually reached fits the previous understanding of the route. When using equipment, steps may be missed, repeated or carried out in the wrong order. The consequences depend on the item: an incorrect setting may prevent the intended communication; a missed fastening may cause an item to be lost. Before a critical step, the current state therefore needs checking rather than relying solely on the plan previously made.
Using available sleep opportunities
A sleep opportunity requires time in which the immediate threat and environment actually permit sleep. A lone evader has to interrupt their own observation to obtain it. Where danger is immediate, limited movement to the next actually reachable visual cover may first be necessary. The threat and the possibility of sleeping are assessed again there.
When a usable opportunity exists, needed sleep and immediately necessary care take priority over work that can wait. Rearranging equipment without a current need, repeatedly going through an already completed plan or making additional journeys uses waking time. If the available time is unpredictable, even a shorter period of actual sleep can be used. Its duration cannot be prescribed for every situation; someone without a clock or a means of waking must also account for being unable to limit a sleep interval reliably.
If other people are actually present, observation and sleep can be agreed between them. It must be clear who is watching which approach and when the task is actually handed over. The person observing also needs sleep; the mere presence of a similarly sleep-deprived companion does not secure the break. Someone alone has to weigh continued observation against the need for sleep using the location and current threat.
Limiting critical tasks
As errors increase, unnecessary simultaneous tasks are dropped. If movement remains necessary and a reachable terrain feature can be recognised, limit the next task: reach that point and reassess there. If paper and a writing implement or a suitable device are available, the essential points and the next operating step can be recorded. Without those means, the task is limited to one immediately executable step. This reduces the burden on memory but does not restore performance impaired by sleep deprivation.
Nodding off or memory gaps are grounds to suspend a dangerous action as far as the situation permits: avoid entering a poorly visible crossing, continuing to drive or beginning a difficult equipment task in that state. From remaining protection, check the present location, the immediately necessary task and the next possibility of sleep. If an immediate threat forces movement, keep the objective limited to the next required step. Additional plans are postponed.
Caffeine, if it is available at all, can temporarily promote alertness. It does not replace sleep or restore every impaired cognitive function. It can also make it harder to fall asleep or stay asleep at the next opportunity. Feeling awake after a stimulant is therefore no more a clearance for dangerous tasks than feeling awake after a work break.
Imposed sleep deprivation in captivity
When guards prevent sleep or repeatedly interrupt it, the affected person cannot plan sleep freely. Sleep loss affects the ability to take in instructions and conversations, remember information and control emotions and behaviour. Individual statements or sequences of events may be remembered less well; irritability can also make dealings with other captives more difficult. Willpower does not restore lost attention and memory performance.
Personal options depend on the control that actually remains. If sleep is allowed, the opportunity should not be consumed by unnecessary self-imposed waking tasks. When a necessary action has to be performed, a simple sequence with one next step at a time helps limit simultaneous demands. Where sleep continues to be prevented, the impairment persists.
After a period of sleep
After sleep, alertness and individual abilities may return without every demanding decision function having recovered equally. Before the next difficult movement or equipment task, the actual situation therefore needs checking again. Further available sleep opportunities remain important, particularly after repeatedly shortened or interrupted sleep.