Sleep

the hat phenomenon

A physiological side-effect of sleep deprivation, also known as the elastic band phenomenon, in which an uncomfortable tightening around the head is felt - as though a hat too small for the head is being worn.

delta waves

Slow, regular brain waves of the highest amplitude and the lowest frequency. These are associated with the latter half of the 3rd and the 4th stages of NREM sleep.

circadian rhythm

The body's natural, inbuilt timing system, or 'body clock', caused by a cyclical production and release of hormones to control bodily functions; these functions dictate the body's sensations of feeling 'drowsy' or 'awake'.

hypnogogic state

The transition period from being awake to being asleep that is characterised by alpha waves, in accordance with the EEG.

microsleeps

Very short periods of drowsiness or sleep that occur while a person appears to be awake.

hypnic jerk

A body spasm due to the relaxation of muscles that may occur during NREM stage 1 of sleep.

K complex

A burst of low-frequency, high-amplitude waves in response to arousing stimuli that may occur during NREM stage 2 of sleep.

sleep cycle

A cyclical pattern of sleep alternating between NREM sleep (and the progression of its four stages) and REM sleep. On average, most adults experience 3-5 cycles per night.

sleep

A naturally occurring altered state of consciousness that is typically spontaneous and primarily characterised by a loss of conscious awareness. The most common ASC.

survival theory

An explanation of the purpose of sleep proposing that sleep evolved to enhance survival by protecting an organism from the perils of the night - making it inactive during the part of the day when it is most dangerous to move about.

theta waves

Brain waves of a moderate frequency and an irregular mixture of high and low amplitudes. These are associated with NREM sleep qstages 1, 2 and 3.

total sleep deprivation

The circumstance in which an individual is deprived of all sleep and kept awake for several days/weeks.

partial sleep deprivation

The circumstance in which an individual receives insufficient/very little sleep for several days/weeks.

sleep debt

The cumulative effect of not getting enough sleep; nightly sleep loss that is owed and needs to be made up.

sleep spindle

A brief burst of high-frequency, low-amplitude brain wave activity during NREM stage 2 of sleep.

sleep-wake cycle shift

A hormonally-induced shift of the circadian rhythm that occurs during adolescence that 'moves' the body clock forward by approximately 1-2 hours. This makes the adolescent sleepier 1-2 hours later than previously felt during childhood. The sleep-wake cycle shift is synced with puberty.

melatonin

A hormone secreted by the pineal gland (located near the hypothalamus in the centre of the brain) that induces sleepiness when there is less detectable daylight. This regulates a person's circadian rhythm.

paradoxical sleep

A reference to REM sleep, in which the body's muscles are relaxed and the body displays few outward signs of movement. Meanwhile, the brain and internal muscles/organs are actively functioning, thus seeming to contradict the body's external appearance.

Stage 2 NREM

A stage of 'light' sleep following stage 1 NREM. During stage 2, body movements lessen, breathing becomes more regular, blood pressure and body temperature continue to decrease, and heart rate is slower (ECG/EKG). Brain waves are predominantly theta waves, however they are slightly lower in frequency and higher in amplitude than the previous stage. Two occurrences specific to stage 2 NREM (as measured by EEG) are sleep spindles (1-second bursts of high frequency waves) and K complexes (peaks of low frequency and very high amplitude). Stage 2 lasts for approximately 10-20 minutes.

restorative theory

An explanation of the purpose of sleep proposing that sleep provides 'time out' for the body to recover from the wear and tear on the body caused during waking time, restore depleted physical and mental energy, repair damaged cells, etc. NREM sleep mainly repairs the body, whilst REM sleep mainly restores the brain and increases neuronal activity, aiding learning and memory. Also known as the recovery theory.

sleep-wake cycle

An individual's circadian rhythm; the rhythmic cycle of sleeping and waking up based on 24-hour intervals.

alpha waves

Brain waves of a high frequency and low amplitude that are slightly slower and larger than beta waves. These are typically associated with a relaxed and wakeful state, daydreaming and the hypnogogic state.

beta waves

Fast, irregular brain waves of a high frequency and low amplitude that are typically associated with normal waking consciousness, focused attention and REM sleep.

NREM sleep

Non-Rapid Eye Movement sleep, in which the brain is active but not as active as during REM sleep or normal waking consciousness (as measured by EEG). Approximately 80 percent of sleep time is spent in NREM sleep. NREM sleep is constituted by four distinct stages that progress from a stage of light sleep (1) to the stage of deepest sleep (4) and back again. Each stage is dominated by a specific identifiable brain wave pattern that differs from the other stages.

REM sleep

Rapid Eye Movement sleep, during which the eyeballs rapidly move beneath closed eyelids (as measured by EOG). Sometimes referred to as a 'fifth stage' of sleep. The brain wave pattern associated with REM sleep is irregular, consisting of low-amplitude, high-frequency beta waves similar to those during normal waking consciousness (as measured by EEG). The heart rate/rhythm is faster and more irregular (as measured by ECG/EKG). REM sleep constitutes approximately 20 percent of our sleep and is the period in which most dreaming occurs. REM sleep has proved to be beneficial to the body and brain and a large part of childhood cognitive development, as it helps to consolidate memories by forming new connenctions between neurons in the brain.

Stage 4 NREM

The 'deepest' stage of sleep; physiological responses are largely similar to those in stage 3, and our muscles are completely relaxed (EMG). Brain activity is dominated by delta waves, with an even slower (low frequency) and larger (high amplitude) pattern than the previous stage (EEG). An individual in stage 4 NREM sleep is practically unaware to their surroundings, and is very difficult to arouse. It is in these stages that phenomena such as sleepwalking, sleep talking and night terrors occur. In the first sleep cycle of the night, stage 4 lasts for approximately 20 minutes. As the night progresses, however, less and less sleep time is spent in stages 3 and 4.

Stage 3 NREM

The beginning of the deepest period of sleep. A stage of 'moderately deep', in which heart rate (ECG/EKG), blood pressure and body temperature continue to drop, and the rate of breathing continues at a slow and steady pace. One is extremely relaxed, and the body is progressively less responsive to the external world. Brain activity is a mixture of slow, large theta and delta waves; with lower frequency and higher amplitude than the previous stage. The presence of delta waves (between 20 and 50 percent of during stage 3 - as measured by EEG) indicating the beginning of slow-wave sleep (SWS). SWS generally begins within an hour of entering the sleep cycle, and will occur for approximately 30 minutes.

Stage 1 NREM

The beginning of the sleep cycle after one transitions from being awake (hypnagogic state) and is truly asleep. We gradually lose awareness of ourselves/our surroundings, however still detect faint sounds in our environment. Physiological responses indicating stage 1 include: a decrease in heart rate (ECG/EKG), respiration, body temperature and muscle tension (EMG). As muscles relax, the body experiences spasms (hypnic jerks). Stage 1 brain activity is characterized by a combination of alpha and theta waves, with more theta waves toward the end of the cycle. Brain waves gradually decrease in frequency with a combination of high and low altitudes (EEG). Stage 1 lasts for approximately 5-10 minutes, but can be as brief as 30 seconds. Daydreaming is also categorised in Stage 1 of consciousness.

polysomnography

The intensive study of a sleeping person involving simultaneous monitoring and recording of various physiological responses during sleep.

slow-wave sleep (SWS)

The period of NREM stage 3 sleep wherein slow, large delta waves begin to function in the brain.

REM rebound

The recovery of REM sleep immediately following a period where REM sleep is lost. This is achieved by spending more sleep time than usual in REM sleep.