Biological Rhythms AO1

Circadian rhythm

• One cycle takes approx 1 day/24 hours

• E.g. Metabolic activity/sleep-wake/Hormones/NTs/Body temperature

Infradian rhythm

• One cycle takes longer than 24 hours

• E.g. menstruation/hibernation/seasonal affective disorder

Ultradian

• One cycle takes less than 24 hours

• E.g. feeding/stages of sleep/alertness

Infradian e.g. menstrual cycle

• 28 day cycle e.g. role of follicle stimulating hormone and luteinising hormone

• First 10-14 days (follicular phase): Oestrogen and progesterone at lowest level =, when bleeding stops, FSH from pituitary gland rises causing growth of ovarian follicles

• Day 14: Ovulatory phase starts, FSH signals ovaries to produce oestrogen, causing LH to surge and releasing egg into fallopian tubes ready for fertilisation etc.

E.g. of ultradian rhythm - sleep stages

• Sleep follows an alternating REM and NREM sleep

• Cycle repeats every 90-100 minutes

• Different stages have different durations

• Stage 1-2 is light, easy to wake, alpha and theta waves, stage 3-4 deep sleep, with slower delta waves and hard to wake. Stage 5/REM sleep = dreaming

• We learn about the sleep stages from recording electrical activity of the brain (using EEGs)

e.g. of circadian rhythm: Sleep-wake cycle

• Approx 24 hours and determines our pattern of wakefulness and sleepiness

• This circadian rhythm also dips and rises at certain times of the day, so our strongest sleep drive usually occurs in two dips (beftween 2-4 am and 1-3 pm, the post-lunch dip)

• Sleepiness is less intense if we had sufficient sleep, and more intense when we are sleep deprived

• Influenced by both internal and external factors called endogenous pacemakers and exogenous zeitgebers

Endogenous pacemakers

• Internal mechanisms governing biological rhythms, in particular the circadian sleep-wake cycle. These can be affected by the environment, despite being biological

• e.g. although sleep-wake cycle continues to function without natural cues from light, research suggests light is required to reset the cycle every 24 hours

• They help maintain regular rhythms in absence of zeitgebers but they are not perfect and need zeitgebers to synchronise the rhythms to our individual behaviours

Exogenous zeitgebers (Siffre)

• Michel Siffre, a French cave explorer spent 6 months in an underground cave in Texas in 1972, separated from natural light/dark cycles

• Allowed him to investigate what happens when the bodily sleep-wake cycle is allowed to ‘free runs away from exogenous zeitgebers. He was wired up so that various bodily functions could be recorded. He ate and slept whenever he wanted

• At first, his sleep-wake cycle was very erratic, but settled down to a fairly regular pattern between 25 and 30 hours, that is slightly longer than the ‘normal’ 24-hour cycle. What was actually 179 days, felt like 151 to him

Exogenous zeitgebers

• External stimuli providing information about elapsed time and prompting changes in bodily activity and the patterns of our biological rhythms

• Classic example of how nature and nurture are both instrumental in shaping our daily or monthly behaviour

• Notably, a key factor in maintaining rhythmic behaviour are the mechanisms underlying the interaction between the EP and EZ, namely lighy