27th December 2005

Sleep Loss

by Mark, Neri & David -- Source: PIA Air Safety Publication

Human operators remain central to safe aviation operations. Fatigue, sleep loss, and circadian disruption created by flight operations can degrade performance, alertness and safety. There are human physiological requirements for sleep, predictable effects on performance and alertness with sleep loss, and patterns for recovery. The circadian clock is a powerful modulator of human performance and alertness and it can be disrupted in aviation through night flying, time zone changes, and day/night duty shifts.

The importance of addressing human-related error, that accounts for approximately 70% of aviation accidents, remains critical to maintaining and improving safety. It is critical that the core human requirement for sleep be managed effectively and operations should reflect the fact that the basic properties of the circadian clock directly affect an operator's performance, productivity, and safety. Fatigue engendered by operational requirement can degrade human performance capability and reduce the safety margin.

Human Sleep Requirements

On average, most humans physiologically require about eight hours of sleep per night though they report usual sleep amounts of about seven to seven and a half hours. When sleep is extended, there is a significant increase in daytime alertness. A Gallop survey examining the report of daytime sleepiness in a random sample of 1,001 individuals demonstrated that 75% reported daytime sleepiness, with 32% of these reporting severe levels. Thirty two percent reported that their sleepiness interfered with activities and 82% of the respondents believed that daytime sleepiness has a negative effect on their productivity.

Effects of Sleep Loss

Sleep loss is common and can be acute or cumulative. In an acute situation, sleep loss can occur either totally or as a partial loss. Total sleep loss involves a completely missed sleep opportunity and continuous wakefulness for about 24 hours or longer. Partial sleep loss occurs when sleep is obtained within a 24 hour period but in an amount that is reduced from the physiologically required amount or habitual total. Sleep loss also can accumulate over time into what is often referred to as "sleep debt". Sleep loss, whether total or partial, acute or cumulative, results in significantly degraded performance, alertness, and mood.

Perhaps the most common occurrence in aviation operations is an acute partial sleep loss or the accumulation of a sleep debt. As little as two hours of sleep loss can result in "impairment of performance and levels of alertness". Therefore, an average individual who obtains six hours of sleep could demonstrate significantly degraded waking performance and alertness. Cumulative sleep loss also significantly reduces alertness and performance. Not only does the sleep loss accumulate but the negative effects on waking performance and alertness also are cumulative and increase over time.

Sleep loss can significantly degrade human performance capability in diverse functions. For example, studies have demonstrated increased reaction time, reduced vigilance, cognitive slowing, memory problems, time-on-task decrements, and optimum response decrements. An important phenomenon, highly relevant to operational environments, is that there is a discrepancy between the subjective report of sleepiness/alertness and physiological measures. In general, individuals will report higher levels of alertness than indicated by physiological measures. Data from an international study of flight crews contains an example where the highest subjective rating of alertness occurred at a time when physiologically the individual was falling asleep within six minutes (an indicator of severe sleepiness).

Recovery from Sleep Loss

There are two factors to consider when determining requirements for recovering from a sleep loss situation. First, when does the internal sleep architecture return to baseline levels. Second, when do waking performance and alertness levels return to their baseline. After sleep loss, recovery is not accomplished through an hour for hour restitution. Rather, recovery is accomplished through an increase in deep sleep observed starting on the first night of regular sleep, though this can be dependent on the duration of the continuous wakefulness. Also, typically two nights of recovery sleep are needed to return to a normal baseline of waking performance and alertness, though this too can be dependent on the length of prior wakefulness.

The Circadian Clock

Beside sleep, the other major physiologic determinant of waking performance and alertness is the internal circadian clock. Circadian (circa=around, dies=day) rhythms fluctuate on a 24 hr cycle with peaks and troughs occurring in a regular pattern. These patterns are controlled by a circadian pacemaker which acts as the timekeeper for a wide range of human functions. One of the most prominent is the 24 hr sleep/wake cycle programmed for a daytime period of consolidated wakefulness and a night time period of consolidated sleep. There are circadian patterns for cognitive and psychomotor performance, physiological activity, alertness and mood.

The trough or low point of the clock is around 3 am to 5 am with many functions demonstrating reduced levels from midnight to 6 am. The lowest level of function (eg alertness, performance, subjective mood, temperature) occur within the 3 am to 5 am trough. Sleepiness has bimodal distribution, showing the most severe low at 3 am to 5 am with a less marked but significant expression, between roughly 3 pm to 5 pm.

Zeitgebers ("time givers") are cues that synchronise circadian rhythms to their 24-hr pattern. To date, light has been demonstrated to be among the most powerful zeitgebers to synchronise the circadian pacemaker. Bright light can dramatically shift the phase of the human circadian clock when applied at responsive times in the 24 hr cycle. Without cues, the intrinsic rhythm of the clock is longer than 24 hrs. Moving to a new light/dark schedule (eg nightwork or time zone change) can create internal and external desynchronization. These involve an internal desynchronization among circadian rhythms and/or a discrepancy between internal timing and external / environmental cues. The internal clock can take from several days to weeks for adjustment, or in some circumstances not fully resynchronize.