Sleep Structure and Homeostatic Sleep Drive

Structure of Normal Sleep

Types and Stages of Sleep

Sleep consists of two main types:

  • Non-Rapid Eye Movement (NREM) sleep: Characterized by a slowdown in breathing, heart rate, and neuronal activity. It is further divided into three stages: N1, N2, and N3.

  • Rapid Eye Movement (REM) sleep: Characterized by rapid eye movements, increased neuronal activity (sometimes exceeding wakefulness), and complete paralysis of postural muscles. This stage is associated with vivid dreaming.

NREM Sleep Stages

  • N1: Lightest stage of sleep, easily disrupted.

  • N2: Stable light sleep.

  • N3: Deep sleep, also known as slow-wave sleep, due to the presence of slow waves of neuronal activity.

Sleep Structure and Hypnogram

The sleep stages across a typical night are represented in a graph called a hypnogram. The hypnogram displays the sleep stages (wake, REM, N1, N2, N3) on the vertical axis and time (e.g., 10 PM to 6 AM) on the horizontal axis.

Typical Sleep Progression

  1. N1: Sleep usually begins with N1 sleep, lasting 1-7 minutes.

  2. N2: Follows N1, lasting approximately 10-25 minutes initially.

  3. N3: Deep sleep, lasting about 20-40 minutes initially.

  4. REM: After N3, a brief period of N2 and body movements may occur before entering REM sleep, which lasts about 10 minutes in the first cycle.

Sleep Cycles

The transition from N1 to N2 to N3 to REM is called a sleep cycle. A typical night includes 4-6 sleep cycles, each lasting approximately 90 minutes on average. The first sleep cycle is 70-100 minutes, while later cycles are 90-120 minutes.

  • REM sleep periods become longer throughout the night. REM sleep dominates in the last third of the night.

  • N3 sleep dominates in the first third of the night and may disappear in later cycles. It is linked to the length of time spent awake.

Arousals and Awakenings

Brief transitions from sleep to waking activity are called arousals, while longer transitions recalled in the morning are awakenings. An average young adult experiences 10-15 arousals per hour of sleep, totaling 80-120 arousals in an 8-hour sleep period. More than 30 arousals per hour can indicate poor sleep quality.

Good sleep quality involves waking less than three times per night and spending 20 minutes or less awake after falling asleep.

Generalizations about Sleep Structure

  • Sleep usually starts with non-REM sleep.

  • Non-REM and REM sleep alternate in cycles lasting 90 minutes on average.

  • N3 sleep dominates the first third of the night, while REM sleep dominates the last third.

  • Less than 5% of the night is spent awake after falling asleep with good sleep.

  • N1 sleep makes up 2%-5% of the sleep period.

  • N2 sleep makes up 45%-55% of the sleep period.

  • N3 sleep makes up 3%-25% of the sleep period.

  • Non-REM sleep typically accounts for 75%-80% of the sleep period.

  • REM sleep typically accounts for 20%-25% of the sleep period.

Homeostatic Sleep Drive

Sleep is regulated by two main mechanisms: the homeostatic sleep drive and the circadian timekeeping system. The homeostatic sleep drive represents our pressure or need for sleep at any given time. As we spend time awake, our pressure for sleep increases until we reach a threshold where we start to feel sleepy.

While we sleep, the homeostatic sleep drive diminishes. If we get enough sleep, this drive returns to baseline levels. If we do not get enough sleep, then we start our day with residual sleepiness. The process ensures that we meet our daily sleep needs.

The drive builds exponentially and then plateaus, representing a building up of pressure for sleep as we spend time awake. Our sleep need on any given day depends on how much sleep pressure we've built up during our previous waking hours.

  • Typically, the longer we are awake, the more sleep we need to recover.

  • We do not need to repay sleep debt hour per hour.

  • Our sleep structure changes to compensate for missing out on sleep.

When we spend more time awake, our pressure for sleep builds until we need sleep to recover from our previous waking day. To recover, we have more N3 sleep, which contains slow ways that reflect the synchronized firing of neurons in the brain. When we've been missing out on sleep, our N3 sleep contains more slow ways than usual, so we essentially have more intense N3 sleep, and this is a way that our brain can recover from missing out on sleep by changing sleep structure rather than needing substantially longer sleep.

In summary, when we miss out on sleep, there are two things that happen:

  • Our sleep structure changes to have more slow ways during N3 sleep and longer periods of N3 sleep.

  • The amount of sleep we need is increased.

Brain Activity

Researchers are still investigating the mechanisms that create the homeostatic sleep drive. The main idea is that there is a substance in the brain that tracks time awake and is degraded during sleep. The most likely candidate is adenosine, a byproduct of the use of adenosine triphosphate or ATP by cells.

Adenosine can slow down the firing rate of neurons in the brain, and it builds up during wakefulness.