WHAT IS SLEEP?

Sleep can be described as an easily reversible stage in which an animal experiences reduced motor activity, diminished response to external stimulation and assumes specific postures (e.g. lying down and closing eyes). It alternates between the state of wakefulness in what is known as a Circadian Rhythm. The periodicity of this rhythm is approximately 24 hours, and it is endogenous, meaning that it exists even without external influences. Sleep occurs in cycles that last approximately 90-110 minutes, repeating 4-6 times per night. These cycles can be divided into two phases: non-REM (Rapid Eye Movement) sleep and REM sleep.

Non-REM Sleep

The non-REM sleep phase occurs at the beginning of each sleep cycle and lasts approximately 70-80 minutes in a Non-REM-REM sleep cycle. This phase is essentially the transition from wakefulness to sleep, and can be observed as 4 distinct stages by their electroencephalogram (EEG) patterns. These 4 stages appear on the EEG with progressively decreasing frequencies and increasing amplitudes. Young adults spend most of their sleep time in stage 2, followed by stages 3 and 4. Stages 3 and 4 of non-REM sleep are characterized by their high amplitude, slow delta waves, and are often referred to as slow-wave sleep. As the Non-REM-REM sleep cycle repeats, the duration of these two stages decreases.

During non-REM sleep, most neurons in the brainstem reduce their activity, while those in the cerebral cortex (forebrain) remain mostly active. These active neurons fire synchronously with a low frequency, creating the high voltage brain waves that can be seen on the EEG. Synchronous brain wave activity also explains why non-REM sleep consumes less energy than wakefulness.

REM-Sleep

REM-sleep (REM means Rapid Eye Movement) refers to the final 5-10 minutes of a Non-REM-REM sleep cycle that are characterized by an organism’s fast eye movements. As these sleep cycles repeat throughout the night, the duration of the REM-sleep phase increases. Unlike non-REM sleep, brain activity (EEG patterns) in this phase closely resembles that of wakefulness. Brain waves are low in voltage, neurons fire independently and energy consumption is high. Motor systems in the brain activate frequently, but no physical movement occurs because neurotransmitters associated with movement are not released. Common occurrences in this phase include vivid dreams, fluctuations in heart rate and breathing, body temperature fluctuation and erections or clitoral enlargement (regardless of dream content).

Sleep regulators

There are many biological regulators of sleep, which generate the various sleep states experienced by organisms. A group of about 100,000 neurons in the base of the forebrain is fully active only during non-REM sleep, so they have been termed “sleep-on” neurons, and are responsible for inducing sleep. Increased body temperature activates some of these neurons, explaining the sleepiness that is associated with hot environments. Further evidence supporting the function of these neurons includes the permanent insomnia produced by lesions in this part of the brain. Similar to “sleep-on” neurons, a specialized group of neurons in the brain stem become more active during REM sleep. They appear to be the cause of the REM sleep phase, so they are called “REM sleep-on” cells.