Circadian Rhythms and Sleep Stages Overview

What is a circadian rhythm?

A rhythm that is about a day in length.

What is the main biological clock in mammals?

The suprachiasmatic nucleus (SCN) of the hypothalamus.

What happens when the SCN is lesioned in rats?

It abolishes the normal 24-hour rhythms of sleep, activity, body temperature, drinking, and steroid secretion.

What bodily functions and hormones show circadian rhythms?

Body temperature, metabolism, blood pressure, hormone levels, and hunger.

When do cortisol and epinephrine levels peak?

They become high shortly after waking and decrease to very low levels right before sleep.

What are ultradian rhythms?

Rhythms shorter than a day, such as the basic rest and activity cycle lasting about 90-100 minutes.

What bodily functions follow ultradian rhythms?

Hormone production, urinary output, alertness, and others.

How does light information reach the SCN?

Through the retinohypothalamic pathway.

What is melanopsin?

A light-sensitive substance found in ganglion cells of the retina.

What hormone does the pineal gland release, and what does it promote?

Melatonin, which promotes sleep.

How do circadian cycles respond to seasonal variations?

The SCN responds to the length of day, and circadian cycles are self-sustaining even in the absence of light cues.

What is the EEG pattern when a person is awake?

A mix of alpha (8-12 Hz) and beta (13-30 Hz) waves.

What characterizes Stage I of sleep?

Low voltage alpha waves, tight brain waves, and the sleeper can be easily aroused.

What occurs during Stage II of sleep?

Progressively deeper sleep with sleep spindles and K complexes appearing periodically.

What characterizes Stages III and IV of sleep?

Slow wave sleep with large, slow delta waves (1-3 Hz) and the sleeper is hard to arouse.

What is REM sleep also known as?

Paradoxical sleep.

What occurs during REM sleep?

Most dreaming takes place, and brain waves are similar to the waking state.

What are PGO waves?

Waves that travel from the pons through the thalamus to the occipital area, initiating the EEG desynchrony of REM sleep.

How does sleep change across the lifespan?

REM sleep makes up 50% of sleep during infancy and decreases until reaching adult levels.

What are common sleep disorders?

Sleep talking, sleepwalking, night terrors, narcolepsy, and insomnia.

What is narcolepsy?

A condition characterized by sudden attacks of sleepiness and quick movement into REM sleep.

What are the functions of dreams?

They may serve as unconscious wishes, information processing, extensions of daily concerns, or problem-solving.

What is the restorative hypothesis of sleep?

Sleep allows the body to repair and replenish cellular components necessary for function.

What is the adaptive hypothesis of sleep?

The amount of sleep depends on food availability and safety considerations.

What role does caffeine play in arousal?

Caffeine acts as an adenosine antagonist, inhibiting the sleep-promoting effects of adenosine.

How do nighttime routines promote sleep?

They help signal the body that it is time to wind down and prepare for sleep.