Sleep and Wakefulness Health Article

Definition

Sleep is a normal state of rest that is characterized by unconsciousness, reduced activity, and limited sensory responsiveness. Sleep differs from other states of reduced consciousness, such as drug intoxication or coma, because it is spontaneous, periodic, and readily reversible. Sleep is usually described by contrasting it with wakefulness, which is characterized by consciousness, sensory responsiveness, and purposeful activity.

Description

Sleep is one of the least understood aspects of human and animal behavior. It occurs in virtually every vertebrate species and seems to be necessary to healthy functioning, but science has been slow to discover how and why sleep occurs. The biological events that take place during sleep are subtle, and many seem to occur at a cellular level within the brain. These events are difficult to observe, and as a consequence our understanding of sleep has developed slowly.

Stages of sleep

Although the sleeping person seems inactive, the sleeping brain exhibits variations in activity throughout the sleep period. Recordings of brain activity, known as electroencephalograms (EEGs), show patterns that occur in a regular cycle lasting about 90 to 100 minutes. This cycle includes relatively brief periods of rapid-eye-movement

(REM) sleep, characterized by back-and-forth movement of the eyes and changes in autonomic nervous system activity. REM is absent in the other phases of the sleep cycle, which are characterized as non-REM (NREM) sleep. Sleep can be divided into five distinct stages based on EEG and REM activity:

• Stage 1 NREM sleep: This lightest stage of sleep occurs as the person is just falling asleep. Stage 1 accounts for about 5% of a normal sleep period.
• Stage 2 NREM sleep: During this period the EEG exhibits characteristic patterns known as "sleep spindles" and K-complexes. This stage accounts for about 50% of a normal sleep period.
• Stage 3 NREM sleep: This stage is characterized by "slow wave" EEG activity, which is associated with deep sleep.
• Stage 4 NREM sleep This stage is very similar to Stage 3, the only difference being the amount of slow wave sleep that occurs. Together, Stages 3 and 4 account for about 20% of a normal sleep period.
• REM sleep. The EEG pattern of this stage is similar to that of Stage 1 NREM sleep. The sleeping person exhibits rapid eye movements and autonomic changes, as well as inactivity of the skeletal muscles. Most dreaming occurs during this stage of sleep. This stage accounts for about 20 to 25% of a normal sleep period.

The first 90-minute sleep cycle of the night begins with Stage 1 NREM sleep and progresses through Stages 2, 3, and 4, ending with a period of REM sleep. Subsequent cycles usually replace Stage 1 with the REM period. In a typical night of sleep, the earlier cycles tend to include more Stage 3 and 4 NREM sleep, with briefer REM periods. As the night progresses, the REM periods tend to get longer while the NREM periods get shorter.

Sleep and biological rhythms

Sleep is one of several biological processes that exhibit a pattern known as a circadian rhythm. A circadian rhythm recurs spontaneously on about a 24-hour cycle. Humans tend to sleep and wake up according to internal circadian rhythms, which seem to be part of our self-regulatory systems.

Circadian rhythms are regulated by a structure in the brain called the superchiasmatic nucleus, which is influenced by exposure to light. Damage to the superchiasmatic nucleus may result in loss of circadian rhythms, however, the individual still exhibits periodic tendencies to fall asleep. This is because a second, homeostatic process also regulates sleep. The individual seems to need sleep after periods of being awake, and the longer the period of wakefulness, the greater the likelihood that the person will fall asleep.