Sleep Series 1: Sleep Physiology

Sleep Physiology

It is important to understand the types and stages of sleep, where restorative functions take place in sleep, and how waking in different stages affects an individual’s immediate level of alertness. Firefighters can use this information to better plan sleeping episodes and naps, build their understanding of sleep physiology, and lay the foundation for the next series of sleep articles and future articles.

Sleep consists of two types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) (see Figure 1). REM sleep is defined “by a complete absence of muscle tone in the muscles that support the body against gravity…”1 NREM sleep is characterized by the “reduced levels of muscle tone, slow regular breathing, no eye movements or slow eye movements, reduced levels of neuronal activity in most brain regions, and high-voltage spindles and slow-wave brain activity.”1 On average, NREM sleep accounts for approximately 75 percent of sleep duration and REM sleep accounts for the remaining 25 percent of sleep.2 NREM sleep further divides into different stages.3,4 Each sleep cycle lasts between 90 to 110 minutes and adults typically experience five cycles in an eight-hour, continuous sleep at night.3,4 Figure 1 illustrates a typical sleep pattern.5

Figure 1: Sleep Hypnogram

In each cycle, the sleeper moves from stages one to three (or four) of NREM sleep. In stage one, sensory stimuli such as light, sound, smell, and touch can easily awake a sleeper. In stage two, waking becomes more difficult. Stage three/four (also known as deep sleep) is characterized by synchronized slow-wave brain wave patterns and is most difficult to arouse. During this stage, sensory stimuli must be more robust and repeated to awaken the sleeper. Thus, firefighters may have trouble waking up during this stage.

A direct correlation exists between NREM sleep stages and level of alertness.3,6 When awakened from stage one, a person is likely to feel awake and refreshed. That same person will experience lower levels of alertness after waking up from stage two. When awakened from stage three/four, the person will experience symptoms of sleep inertia (e.g., feeling of grogginess and low alertness7).

The effects of sleep inertia divide into two areas: subjective alertness and cognitive performance.8 Some researchers found that the effects from sleep inertia could last up to four hours.7 Others found that subjective alertness levels returned to normal after 40 minutes and cognitive performance levels returned to normal after 70 minutes.8 Furthermore, the effects of sleep inertia tend to increase in sleep-deprived individuals.7 They also noted that lasting effects rarely exceed thirty minutes in non-sleep-deprived individuals.

Firefighters’ ability to maintain situational awareness and rapidly make decisions may be significantly impaired when experiencing sleep inertia and low levels of alertness . Furthermore, as alertness, reaction time, and cognitive performance9 typically decreases throughout the day and night, firefighters may become further susceptible to a higher risk of injury or accident.

An implication of this research for firefighters is that they can better structure their naps. For example, by understanding the timing of sleep cycles and stages, firefighters can avoid waking in deeper stages of NREM (where the effects of sleep inertia are greater). Using Figure 1 as a guide, we can see that an average nap duration of 20 minutes usually would not be great enough to progress into deep sleep. Thus, a firefighter can quickly obtain the benefits of a “power nap” while limiting the negative effects of sleep inertia and low alertness. On the other hand, a nap duration of approximately 90 minutes is usually great enough to complete one cycle of REM and NREM sleep. Thus, allowing the firefighter to receive some restoration in deep sleep, waking in a lighter stage, and feeling refreshed.

For fire chiefs, understanding these aspects of sleep are critical in deciding department policy. Using evidence-based research will help justify decisions that are best for firefighters and the communities they serve.

The next series will discuss sleep recovery: what is recovery, the challenges firefighters face, and steps to improve recovery.

  1. Carskadon, Mary. "Encyclopedia of Sleep and Dreaming." 498. New York: Macmillan Publishing Company, 1993.
  2. Majde, Jeannine A. and James M. Krueger. 2005. "Links between the Innate Immune System and Sleep," Journal of Allergy and Clinical Immunology 116(6):1188-1198. doi:
  3. Joffe, Mark D. 2006. "Emergency Department Provider Fatigue and Shift Concerns," Clinical Pediatric Emergency Medicine 7(4):248-254. doi:
  4. Luyster, Faith S., et al. 2012. "Sleep: A Health Imperative," Sleep 35(6):727-734. doi:10.5665/sleep.1846
  5. Gonnissen, H. K. J. 2013. The Broken Cycle of Sleep: Energy Balance through Circadian Alignment: Maastricht:, Uitgeverij BOXPress.
  6. Williams, Harold L, et al. 1973. "Physiological Psychology: Sleep," Annual Review of Psychology 24(1):279-316. 
  7. Tassi, Patricia and Alain Muzet. 2000. "Sleep Inertia," Sleep Medicine Reviews 4(4):341-353. doi:
  8. Jewett, Megan E., et al. 1999. "Time Course of Sleep Inertia Dissipation in Human Performance and Alertness," Journal of Sleep Research 8(1):1-8. doi:10.1111/j.1365-2869.1999.00128.x
  9. Duchon, James C. and Thomas J. Smith. 1993. "Extended Workdays and Safety," International Journal of Industrial Ergonomics 11(1):37-49. doi:
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