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CoursesPhysiologyNervous systemCerebral cortex and higher cognitive functionsSleep and wakefulness

CoursesPhysiologyNervous systemCerebral cortex and higher cognitive functionsSleep and wakefulness

Sleep and wakefulness

Learning objectives:

After completing this study unit, you will be able to:

Define and describe the stages of sleep.
Compare the key electroencephalographic and physiological changes during the different stages of sleep.
Describe the sleep cycle and changes with age.
Discuss the sleep-wake cycle and explain the role of the circadian rhythm.

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Sleep is a temporary state of unconsciousness from which one can be awakened or aroused by external or internal stimuli. It can be categorized into nonrapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.

During the different stages of sleep (N1, N2, N3 and REM) and wakefulness, the electrical activity of the brain can be recorded on an electroencephalogram (EEG). The brain waves seen on the EEG are classified into gamma, beta, alpha, theta and delta waves, and can vary in frequency and amplitude.

NREM and REM sleep alternate every 90 to 120 minutes over the course of a 6 to 8 hour sleep period. The duration and number of sleep cycles vary with age.

The sleep-wake cycle is a 24 hour pattern of sleep and wakefulness. The increasing need to sleep is due to a homeostatic sleep drive, which is countered by the circadian wake drive. The circadian rhythm regulates multiple physiological functions (body temperature, sleep, hormone secretion) throughout day and night, and is governed by the suprachiasmatic nucleus in the hypothalamus.

Learn more about the physiology of sleep in this video!

Explore concepts

Sleep stages

A 6-8 hour period of sleep can be classified into different stages, each characterized by brain waves with different frequencies and amplitudes. Learn more about these topics in the images below.

Circadian rhythm

The circadian rhythm regulates our sleep and wakefulness cycle. Explore this topic in more detail below.

Multiple areas of the brain are involved in sleep and wakefulness, take a look at some important ones in this image gallery!

Take a quiz

Grab a cup of coffee and test yourself with this quiz on sleep!

Summary

Key facts about sleep and wakefulness
Definition of sleep	A temporary state of unconsciousness from which one can be awakened by external or internal stimuli
Types of sleep	Nonrapid eye movement (NREM) sleep Rapid eye movement (REM) sleep
Stages of sleep	NREM sleep: N1, N2 and N3 stages REM sleep: R stage
Brainwaves (EEG waves)	Recorded by electroencephalography; Awake and alert with eyes open: beta waves Awake and relaxed with eyes closed: alpha waves N1 NREM sleep: theta waves N2 NREM sleep: theta waves, sleep spindles, K complexes N3 NREM sleep: delta waves REM sleep: beta-like waves
Sleep cycle	NREM and REM sleep alternate every 90-120 minutes
Sleep-wake cycle	24 hour pattern of sleep and wakefulness Balance between the homeostatic drive for sleep and the circadian wake drive Melatonin and cortisol play an important role in this cycle
Sleep-promoting areas	Ventrolateral preoptic nucleus and median preoptic nucleus of the hypothalamus Neurotransmitters: GABA, galanin
Wake-promoting areas	Reticular formation of the brainstem, hypothalamus (lateral hypothalamic area, tuberomammillary nucleus), basal forebrain Neurotransmitters: glutamate, acetylcholine, orexins, norepinephrine, serotonin, histamine
Circadian rhythm	Natural 24 hour cycle of physiological functions Governed by the suprachiasmatic nucleus in the hypothalamus, which responds to the presence or absence of light Regulates melatonin secretion by the pineal gland

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