text: Chapter 11
  during the nighttime hours. Normal sleep and alertness are typically not restored in the jet-lagged brain for
  several days—the time it takes for the brain’s internal clock to resynchronize to the day-night cycle of the
  new, local time zone. (See paragraphs 11-34 through 11-54 for strategies to mitigate the effects of disruptions
  in normal sleep patterns.)
  11-5. Maintaining a consistent sleep-wake schedule on both duty and non-duty days has the benefit of
  strengthening and reinforcing the internal wake- and sleep-promoting processes controlled by the brain’s
  internal clock. These processes constitute the “circadian rhythm of alertness.” Individuals who maintain
  consistent sleep-wake schedules (especially on arising at the same time each morning and experience their
  first exposure to daylight at the same time each day) derive the maximum benefits from the circadian rhythm
  of alertness, with well-consolidated sleep at night and optimum alertness during the daytime. A consistent
  and regimented schedule of sleep- and wake-related activities helps to lock in other biological systems
  associated with circadian rhythms. These include hormone release, digestion, muscle strength, and
  cardiovascular performance. Circadian rhythms act in tandem with the need to sleep which builds throughout
  a day. These rhythms optimize the process of falling asleep, staying asleep, and ensuring quality sleep.
  11-6. However, Soldiers can only achieve such benefits if they get adequate sleep (for most individuals, 7 to
  9 hours of sleep per night) on a regular basis. If they get less sleep (for example, 6 hours per night during the
  duty week), then a “sleep debt” accrues. In such cases, it is better to sleep in on off-duty days and pay down
  the sleep debt, rather than sacrificing sleep to try to maintain a consistent sleep-wake schedule and strengthen
  the circadian rhythm of alertness.
  11-7. Although the circadian rhythm of alertness generally promotes a 24-hour cycle of daytime wakefulness
  and nighttime sleep, there is also a temporary afternoon “dip” in alertness. This dip becomes especially
  noticeable in individuals who have a significant sleep debt (for example, not regularly obtaining adequate
  sleep). For those able to take advantage of it, the afternoon dip provides an opportunity for obtaining good
  quality daytime sleep to help pay down any existing sleep debt. Soldiers can generally take these naps without
  significantly disrupting the circadian rhythm of alertness—provided that the naps are not so long or so
  frequent that they begin to impair the ability to initiate sleep at night.
  SLEEP CONTINUITY
  11-8. The restorative value of sleep is determined not only by the duration of the sleep period, but also by
  the continuity of the sleep period—that is, the extent to which the sleep period is continuous and
  uninterrupted. The sleeping brain cycles through non-rapid eye movement (known as NREM) and rapid eye
  movement sleep (known as REM or “dreaming sleep”) every 90–120 minutes. The full benefit of sleep occurs
  when the brain completes 4–5 complete cycles. Non-rapid eye movement sleep makes up most of the first
  half of the night’s sleep. During this type of sleep, the body releases hormones that help repair and rebuild
  muscles and replenish energy. There are three stages of non-rapid eye movement sleep: stage 1 (the lightest
  sleep stage), stage 2 (which accounts for approximately 50 percent of nighttime sleep), and stage 3 (the
  deepest and the most recuperative sleep stage). During sleep, the body clears toxins that have accumulated
  throughout the day (as by-products of healthy brain functioning) from the brain; it also fixes and transforms
  new memories into usable knowledge. Both types of sleep are essential, and it is important that the brain
  cycles appropriately between non-rapid eye movement and rapid eye movement sleep across the night. When
  sleep is interrupted or shortened, natural progression of sleep cycles are disturbed reducing the beneficial
  effects of the sleep.
  SLEEP READINESS FUNDAMENTALS
  11-9. Like the rest of the body (for example, muscles, skin, and liver), the brain has physiological needs for
  food, water, and oxygen—basic needs that must be met not only to ensure proper brain functioning, but to
  sustain life itself. However, unlike the rest of the body, the brain has one additional physiological need: sleep.
  The brain requires sleep to maintain normal function. Sleep is necessary to sustain not only alertness, but
  also higher order cognitive abilities such as judgment, decision making, and situational awareness. In short,
  sleep makes Soldiers better at being Soldiers.
  11-10. The brain needs sleep to restore and repair itself, to work efficiently, to fix new memories, and to
  process new information appropriately. Sleep also clears away waste products from normal breakdown of