Circadian sleep-wake cycle

How long does is the circadian cycle?

24 hours

What is the circadian rhythm mainly controlled by?

SCN

Internal pacemaker

• SCN

• pineal gland

SCN

• group of neurons

• located in hypothalamus

• body’s internal clock

Research into internal pacemaker

• Decoursey et al: chipmunks

• Ralph et al: hamsters

Decoursey et al

• studied chipmunks in natural habitat

• 30 SCN lesion, 24 sham surgery, 20 controls

• higher proportion of SCN lesion killed by weasels

• SCN damage → increased nocturnal activity → more vulnerable to predators

• CONCLUDE: SCN crucial for regulating sleep-wake cycle

Ralph et al

• investigate if SCN controls circadian rhythm in mammals

• hamster w/ genetic mutation → 20 hr circadian cycle

• SCN from abnormal hamster transplant to normal 24h hamster, compare to another normal hamster

• hamsters that received abnormal SCN, developed 20h cycle

• CONCLUDE: SCN regulates circadian rhythm

Pineal gland & melatonin

• pineal gland produces melatonin

• melatonin = hormone that induces sleepiness

• melatonin increase, serotonin decrease

Melatonin pills

• early morning = delayed sleep

• afternoon = earlier sleep

• evening = little effect (melatonin already high)

Siffre’s cave study

• 2 months underground, no sunlight, no clocks

• sleep-wake cycle became free-running at around 24h 30m

• shows internal pacemakers run without zeitgebers, but slightly longer than 24h

Aschoff & Wever

• Ps stayed in underground WW2 bunker 3-4 weeks, no natural light

• 85% adopted cycles of 25-27h

• free running rhythm is slightly longer than 24h and needs light to synchronise

Skene & Arendt

• studied blind people

• partially sighted (some light perception) → normal rhythms

• fully blind (no light perception) → irregular rhythms, poor sleep

• light is a dominant zeitgeber

Real life applications

• shift work = night shift cause circadian disruption → fatigue, poor concentration (artificial light reduce disruption, but doesnt fully reset rhythms)

• jetlag = crossing time zones cause phase shifts → body cannot adapt quickly (gradual sleep schedule adjustment before travel can help)