Circadian rhythms

Pattern of behaviour occurs/recurs approx every 24 hours, set + reset by environmental light levels.

Light + darkness - external signals, determine when feel need to sleep + when to wake. Dips + rises at diff times of day, strongest sleep drive occurs in 2 'dips' - b/ween 2am-4am + b/ween 1-3pm. Sleepiness during dips less intense if had sufficient sleep, more intense when sleep deprived.

Sleep + wakefulness not detemined by circadian rhythm alone - homeostatic control. When awake for long time, homeostasis tells us need for sleep increasing b/c energy used. Drive for sleep increases during day.

Circadian rhythm eeps us awake as long as daylight, homestatic system makes us sleepier whether light or day. Internal circadian clock - free-running. CR intolerant of major alterations in sleep + wake schedules b/c causes bio clock to be out of balance.

Core body temp - sleep occurs when core te,p behind to drop, start to rise during last hours of sleep - feeling of altertness. Small drops of temp occurs b/ween 2pm-4pm.

Hormone production - prouction + release of melotonin follows CR - peak levels during hours of darkness. By activating chemical receptors in brian, melatonin encourages feelings of sleep. When dark, more M produced, when light, production of M drops - person wakes.

Hughes (1977) - Antarctic summer, cortisol levels followed familar pattern. 3 months continuous darkness, pattern changed, peak cortisol levels at noon - when men now awoke. Light important in maintaining CR.

Individual diffs - cycle length - can vary from 13 to 65 hours. Duffy et al (2001) - 'morning people' + 'evening people' - peak at diff points.

Buhr et al (2010) - temp more important than light. Body temp fluctuates on 24 hour circadian rhythm, small changes can send powerful signals to body clocks.