DP 2 sleep wake pattern regulation

Circadian Rhythms

• Circadian rhythms are biological processes that follow an approximately 24-hour cycle and regulate the sleep-wake cycle.

• They influence:

  • Alertness

  • Body temperature

  • Hormone release, including growth hormones

  • Sleepiness and wakefulness

• Alertness is generally highest during the day and lowest in the early morning.

• Our natural biological clock can run slightly longer than 24 hours (up to about 25 hours), but environmental cues help synchronise it to a 24-hour day.

Zeitgebers

• Zeitgebers are external environmental cues that reset or synchronise our biological rhythms.

• The most important zeitgeber for the sleep-wake cycle is light.

Melatonin

• Melatonin is a hormone involved in initiating sleep and regulating the sleep-wake cycle.

• It is produced by the pineal gland.

• Darkness → more melatonin → increased drowsiness.

• Light → less melatonin → increased alertness.

• Melatonin levels remain high during the night and decrease during the day.

Suprachiasmatic Nucleus (SCN)

• The SCN is located in the hypothalamus and acts as the body's main biological clock.

• It receives information about light from the eyes.

• Morning light → SCN inhibits melatonin → wakefulness.

• Darkness → SCN signals pineal gland to release melatonin → sleepiness.

Artificial light

Artificial light from room lighting and electronic devices can reduce or delay melatonin release, potentially making it harder to fall asleep.

Application: Jet lag

• Jet lag occurs when the body's internal clock is out of sync with the new time zone.

• Morning sunlight can act as a zeitgeber to help reset the circadian rhythm to the new time zone.

Ultradian Rhythms

• Ultradian rhythms are biological cycles that occur more than once within 24 hours.

• Examples include:

  • Heartbeat

  • Respiration

  • Hunger

  • Sleep stages

• They are repetitive and predictable.

Sleep as an ultradian rhythm

During sleep, we repeatedly cycle through:
NREM1 → NREM2 → NREM3 → REM → repeat

• One complete cycle lasts approximately 90 minutes.

• Most adults experience around 4–6 cycles per night.

• Sleep stages can be measured using:

  • EEG → brain activity

  • EOG → eye movements

  • EMG → muscle activity

NREM 1

• Lasts approximately 5–10 minutes.

• Transitional stage between wakefulness and sleep.

• Hypnagogic state occurs before/around falling asleep.

• Hypnic jerks may occur.

• Very easy to wake.

• Eyes move slowly.

• Muscles relax.

• Heart rate and breathing decrease.

NREM 2

• Approximately 20 minutes per cycle and makes up a large proportion of total sleep.

• Deeper than NREM1 but still relatively easy to wake.

• Eye movement decreases.

• Muscle activity decreases.

• Heart rate and breathing continue to slow.

• Characterised by:

  • K-complexes → high-amplitude brain waves that may help prevent arousal.

  • Sleep spindles → brief bursts of electrical activity; associated with being truly asleep and potentially learning/memory.

NREM 3

• Deepest stage of NREM sleep.

• Also called slow-wave sleep/delta sleep.

• Makes up approximately 10–15% of sleep.

• Usually lasts around 30 minutes initially and decreases as the night progresses.

• Very difficult to wake → highest arousal threshold.

• Very low conscious awareness.

• Little/no eye movement.

• Very low muscle activity.

• Heart rate and breathing are at their slowest and most regular.

• Body temperature is lowest.

• Sleepwalking and sleep talking can occur.

Remember: NREM3 combines what older classifications called NREM3 and NREM4.

REM Sleep — “Paradoxical Sleep”

REM is called paradoxical sleep because the brain is highly active while the body is almost paralysed.

• First REM period lasts about 10 minutes and becomes longer throughout the night.

• Dreaming commonly occurs and dreams are more likely to be recalled if awakened during REM.

• Rapid eye movements occur → detected by EOG.

• EEG brain activity looks similar to an awake brain.

• Heart rate, breathing and blood pressure increase and fluctuate.

• Body temperature becomes more closely related to the surrounding environment.

• Very little muscle tension → near-total muscle paralysis.

How Sleep Changes During the Night

Early night

More NREM3 (deep sleep) + less REM

Later night

Less NREM3 + more REM

Overall, approximately:

• 80% NREM

• 20% REM

The sleep cycle does not remain identical throughout the night. As the night progresses, deep NREM sleep decreases while REM periods become longer.