Napping

Rest breaks and napping are fatigue countermeasures that have received some study. The evidence that naps improve performance appears to be more consistent than the evidence that rest breaks improve performance. According to Luna (1997), "there is a relatively large body of literature documenting the beneficial effects of naps taken during and prophylactically before, continuous operations." For example, Harma (1989) showed that persons taking prophylactic naps were more alert on the night-shift than those that did not. Worksite napping strategies are used in some international air traffic control operations (Costa, 1993). Naps have been shown to be effective in improving performance and reducing subjective sleepiness, especially if they are taken before a prolonged work period. Dinges (1994) noted the effects of "prophylactic" napping and also found that naps of only "25 minutes" could improve performance for hours afterwards.

Prophylactic naps should be distinguished from replacement or maintenance naps. Prophylactic naps are taken prior to a period of sleep loss (e.g. prior to an initial night shift). Replacement naps are taken after a period of sleep loss. Maintenance naps are a form of replacement naps that occur on duty. Rosekind, Graeber, Dinges et al (1994) demonstrated the beneficial effects of strategic on-duty napping with airline flight crews in maintaining alertness and performance.

Dr. Moore-Ede in The 24 Hour Society suggests that a 10-15 minute nap is "ideal" for sleepy drivers. He recommends avoidance of longer naps which "can leave you worse off than before." Ferrer et al (1995), on the other hand, recommends either a 30-minute or 3-hour nap as the "best times for short naps."

Sleep Inertia

Sleep inertia or grogginess after awakening is "ubiquitous" and occurs after awakening from sleep at anytime of the 24-hour day or after a nap. (Dinges, 1989). In some studies naps have been followed by a period of "sleep inertia" immediately after awakening. Sleep inertia can be brief lasting from 1-5 minutes in non-sleep deprived subjects. However, sleep inertia can last longer if the nap follows a prolonged period of wakefulness. The duration of these effects however, is usually "so brief that they are ignored in many investigations of the effects of naps in adults." (Dinges, 1989, p. 195) Never the less, "sleep inertia can be severe if the nap is taken by someone with a severe pre-nap sleep debt." (p. 50) The degree of sleep inertia reported appears to be correlated with the time of day that the nap is taken and the length of time since the last major sleep period. Interestingly, reaction time performance was directly related to sleep stage at awakening. Persons awakened during stage 4 sleep (the deepest) yielded maximum reaction times (Akerstadt, Torsvall, and Gillberg, 1989).

This grogginess can have a negative influence on performance lasting as long as 20 to 30 minutes (Taub et al, 1977; Taub, 1979). Dinges (1995) reported that the advantages of napping are not realized until the negative effects of sleep inertia are overcome. However, sleep inertia can usually be reversed within 15 minutes by activity and noise. Some studies suggest that sleep inertia may last anywhere from 5 minutes to up to one hour after awakening. Other researchers suggest that sleep inertia typically lasts 15 to 30 minutes after awakening. One researcher recommended that napping aircrews be monitored for 10 minutes after awakening until sleep inertia passes (Ferrer et al, 1995). It should be noted that most reports of the effects of sleep inertia are found in sleep deprivation studies in which study participants had been awake for long periods of time. Rosekind, et al (1996) suggests limiting naps that are immediately prior to work to 45 minutes in order to minimize the chance of entering deeper sleep.

Deep, slow wave sleep has also been associated with sleep inertia. The degree of sleep inertia appears to be correlated with the timing of the nap, the depth of sleep and the length of prior wakefulness. Sleep inertia has been shown to increase when associated with naps after long periods of prior wakefulness, naps taken during the first few hours of nocturnal sleep or during the circadian trough, and with deeper (slow wave) sleep.

Not withstanding these negative effects, Dinges (1985) indicates that sleep inertia can be anticipated and planned for if a worker must wake quickly and respond to immediate performance expectations. More importantly, Rosekind (1997) noted that in an emergency situation the effects of adrenaline can quickly overcome the negative effects of sleep inertia.

Napping has also been associated with other negative effects. The Office of Technology Assessment’s report on Biological Rhythms and Work Schedules cautions shift workers to avoid off-duty naps if the next major sleep period is at night. Off-duty napping may interfere with a permanent night worker’s ability to sleep normally. Rosa et al (1990) hypothesized that napping may hinder adaptation to shift work by "providing an excuse for sacrificing regular sleep, by making a person too drowsy upon awakening, or by slowing the inversion of the circadian rhythm." The latter concern may be less important for rotating shift workers or permanent shift workers who often do not maintain the same sleep schedule on days off as on workdays. Torii et al. (1982) in a study of nurses on occasional night shifts found that naps taken before and after night work reduced the total sleep time during the subsequent night’s sleep. Long-term effects of naps on performance, alertness and adaptation to shift work have not been defined. One study (Bonnet and Arand, 1994) indicated that individuals who had several one-hour naps during night work experienced larger performance decrements than those who had a prophylactic four-hour nap and remained awake all night.

Napping and Performance

According to Dinges (1989) there have been more than a dozen studies investigating the effects of napping on mood, performance and psychophysiological activity. Mood variables and self-reported alertness have improved in most studies. One study observed a statistically significant improvement in performance in the hour after a nap. (Taub, et. al 1976). Other studies did not find such an effect. However, all of these studies involved subjects who had taken either a morning or an afternoon nap following a normal night’s sleep. Several studies reviewed by Dinges (1989) reported improved performance several hours after the nap and can have a positive effect as much as 10 hours after a nap. Helmus, Rosenthal, Bishop, Roehrs, Syron, and Roth (1997) reported that sleep deprived normal volunteers were more alert after a 120-minute nap than were narcoleptic patients similarly sleep deprived.

A recently published study by Gillberg, Kecklund, Axelson, and Akerstedt (1996) investigated the effects of a 30-minute daytime nap on alertness. After one normal night’s sleep participants received only 4 hours sleep and were tested every hour thereafter. After 10 hours awake EEG sleepiness and subjective sleepiness increased while vigilance performance decreased. A short nap (mean 19.8 minutes, standard error 2.8 minutes) brought performance back to baseline levels.

Another recent study by Reyner & Horne (1997) found that a short nap of less than 15 minutes (mean =12.4 minutes, standard error =1.2 min) combined with a cup of coffee containing 200 mg of caffeine reduced the number of traffic "incidents" in a driving simulator "3-4 fold." There was no evidence of "sleep inertia" reported. This may have been due to the lack of an accumulated sleep debt.

Most studies of short on-duty naps may not be directly applicable to operational environments and to shift work. An exception is the NASA Ames Research Center study on planned cockpit napping. (Rosekind et, al, (1994). The study demonstrated that on-duty naps, averaging 25 minutes in length, improved performance and alertness in aircrews on long-haul flights. The Ames study appears to be well designed and does support on-duty napping as a promising intervention in a controlled operational setting. Dinges (1995) commented however, that the Rosekind (1994) report demonstrated five "fundamentally important points about using planned napping as a fatigue countermeasure strategy":

  1. It was possible to safely and effectively plan ahead of time for when a nap would be taken
  2. It was possible for every operator to fall asleep in a reasonable period of time
  3. Sleep inertia did not pose a serious problem because 20 minutes was allowed for its dissipation prior to assuming duties
  4. As in laboratory studies the nap improved objective measures of alertness … but did not eliminate feelings of fatigue
  5. The beneficial effects of a single nap were most evident on night flights, when control crews showed increasing fatigue relative to crews allowed a nap. (p. 51)

In conclusion, several summary points about napping can be made:

In general, the effects of napping, following the elimination of sleep inertia, have positive effects on performance that can be seen as long as 10 hours after a nap has been taken. However, it appears that napping research that utilizes a methodology readily generalizable to the on-duty activity of railroad employees is scarce. Thus, definitive conclusions about the effects of napping on the actual day-to-day performance for railroad employees are premature. Further study of the duration and timing of naps in the work/rest cycle, and sleep inertia is needed to clarify the best utilization of this technique.