biological rhythms

• Biological rhythms are processes in the body that follow a cycle
•  Circadian rhythms are biological rhythms that repeat every 24 hours. Examples include the sleep-wake cycle and variations in body temperature.
• Body temperature follows a circadian rhythm across a 24 hour cycle. Our lowest body temperature is around 4am. Our highest body temperature is around 6pm.
• Chronotherapeutics was developed using knowledge of the circadian rhythm.  Chronotherapeutics allows people to take drugs which then aren’t effective until necessary, hours later.
• Research support for individual differences in circadian rhythms. Duffy 2001 conducted a month-long controlled observation and found individual differences in the timing of people’s circadian rhythms which are accompanied by individual differences in the timing of other circadian rhythms, like body temperature. Morning people wake at 6am and sleep at 10pm, evening people wake at 10am and sleep at 1am. This demonstrates there may be innate individual differences in circadian rhythms
• Ultradian rhythms are biological rhythms that repeat more than once every 24 hours. Examples of ultradian rhythms are appetite and stages of sleep
• In stage one of sleep, we are just starting to fall asleep and can be easily woken up.
• In stage two of sleep, our heart rate starts to slow down and our temperature starts to drop.
• In stage three and four of sleep, we fall into a deeper and deeper sleep.
• In REM stage of sleep, we are sleeping deeply and our eyes move rapidly and our brain is active. It is during this stage of sleep that we often dream.
• The basic rest activity cycle (BRAC) is a 90-minute ultradian rhythm of alertness that we cycle through in a day. BRAC helps explain why professional musicians and athletes break up their practice into short segments.
• Research support for individual differences. Tucker et al 2007 conducted a controlled observation into people's stages of sleep. Found that there were individual differences in the amount of time people spent in each sleep stage and the biggest individual difference in amount spent in different sleep stages was during deep sleep. This demonstrates that there may be innate individual differences in ultradian rhythms
• Infradian rhythms are biological rhythms that repeat less than once every 24​ hours. Examples include cycles like menstruation and hibernation
• The menstrual cycle controls a woman's fertility and ovulation and the release of oestrogen and progesterone across a 28​ day cycle.
• Seasonal affective disorder is an infradian rhythm that repeats every year. It is when people experience low mood during winter and better mood during summer.
• Individual differences in infradian rhythms. The menstrual cycle is typically 28 days, however, there is considerable variation between women. It varies between 23​ and 36​ days in adult women and 21​ and 45​ days in teenage girls. This demonstrates that there may be innate individual differences in infradian rhythms
• Pacemakers are structures in the body that control the timing of biological rhythms.
• The pineal gland controls our sleep/wake cycle by releasing melatonin into the bloodstream. The more melatonin in the bloodstream, the more sleepy we feel.
• The pineal gland increases melatonin release at night to make us sleep and reduces melatonin release in the day so that we feel awake and alert.
• At the start of the day, the pineal gland decreases melatonin release. This means that the sleep centres stop being active and the wakefulness centres are released from inhibition and become active again. People become more likely stay awake because they have less melatonin.
• The suprachiasmatic nucleus is a group of neurons in the hypothalamus that controls the pineal gland through nerve impulses.
• The suprachiasmatic nucleus tells the pineal gland when to release melatonin.
• Research support for endogenous pacemakers. Ralph et al conducted a study investigating the sleep/wake cycle in hamsters. The control group had a normal sleep/wake cycle and the experimental group had a 21 hr sleep/wake cycle. Found both groups of hamsters lost their sleep/wake cycle when their SCN was removed and adopted the sleep/ wake cycle of the other group. This is positive as it found the role of endogenous pacemakers is crucial for the sleep/ wake cycle. A limitation of Ralph's hamster research is that it is based on hamsters so results may not be generalisable to humans.
• The suprachiasmatic nucleus is a set of neurons, located in the hypothalamus that controls the pineal gland with electrical impulses
• The SCN tells the pineal gland to release melatonin, which activates the sleep centre and inhibits the wakefulness centre.
• The suprachiasmatic nucleus is an endogenous pacemaker.
• Structures within the body that control biological rhythms are called endogenous pacemakers.
• Stimuli outside the body that influence biological rhythms are called exogenous zeitgebers.
• Examples of exogenous zeitgebers for the sleep/ wake cycle are light, noise and social customs
• Light helps the pacemakers stick to a regular 24​ hour cycle. It is initially picked up by sensory receptors in the retina of the eyes which communicate with sensory neurons. Sensory neurons communicate with relay neurons that travel to the visual cortex.
• Sensory receptors that detect light can send information directly to the SCN to influence melatonin release.
• Without light, the pacemakers would overrun, stretching out the sleep/wake cycle to a 25/26 hour cycle.
• Research support for exogenous zeitgebers. Siffre's cave study was a case study that researched the role of light as an exogenous zeitgeber. Siffre isolated from natural light and lived in a cave for 6 months with no access to exogenous zeitgebers. Siffre found his sleep/wake cycle increased to a 25-30 hour cycle. This shows that exogenous zeitgebers are essential to synchronise our circadian rhytms.
• Further support comes from Aschoff & Weber. Aschoff & Weber 1966 made participants sleep in a bunker with no natural light. Participants had access to lamps. Aschoff & Weber found without access to natural light, participants’ sleep/wake cycles lengthened to around 25−27 hours. This demonstrates how exogenous zeitgebers are essential in synchronising circadian rhythms
• A limitation of Siffre's cave study and Aschoff & Weber is that participants had access to artificial light which may have helped to reset their circadian rhythms. This is problematic as the ability of endogenous pacemakers to control the sleep/ wake cycle may have been overestimated.
• When we travel to a new timezone, our endogenous pacemakers continue to operate in the old timezone, while our exogenous zeitgebers operate in the new timezone. They become desynchronised. This is jetlag.
• Bright light therapy is exposure to artificial light which helps people reset their circadian rhythms to the new timezone before they travel. A few days before people travel on a long distance flight, they’re exposed to artificial bright light to recreate the light levels in the place they’re flying to. This prevents jetlag
• Support for exogenous zeitgebers in menstrual cycle. Reinberg 1967 investigated whether light could act as an exogenous zeitgeber for the menstrual cycle. He conducted a case study on a female participant who lived in a cave for 3 months with no exogenous zeitgebers. Her menstrual cycle shortened to 25.7 days. This supports the idea that light helps to control and reset the menstrual cycle, which is an infradian cycle. However, as this is a case study, we cannot be sure how these findings generalise to the wider female population.