Circadian rhythms

• Circadian rhythms tend to be patterns of behaviour that reoccur every 24 hours
• They’re set and reset by a combination of internal and environmental cues
• An example of a circadian rhythm is the sleep/wake cycle
• This circadian rhythm determines when we should be awake and asleep and impacts heavily on our levels of alertness throughout the day
• In addition, other circadian rhythms are closely linked to this, for example,

  the release of the hormone melatonin from the pineal gland
• Peak levels of melatonin occur in the hours of darkness. When it’s dark, melatonin activates chemical receptors in the brain, causing feelings of fatigue and sleepiness, when it starts to get light, melatonin levels decrease and we start to feel more alert and awake
• Micheal Siffre (1975) spent extensive periods underground to study the effect this had on his own biological rhythm. In the isolated cave environment, his internal body clock was allowed to ‘free-run’. The usual cues a person would experience would be natural light
• In this case, Siffre spent six months in a cave with no natural cues about the date or time. The only thing influencing his behaviour was his internal body clock, which without any external cues was a ‘free-running’ circadian rhythm. During this time, Siffre settled into a sleep/wake cycle of between 25 and 30 hours. This evidence suggests that natural light sources in the environment are vital to keeping an individual to a 24-hour cycle
• it was assumed that only natural light has an influence on our biological rhythms; however, research by Czeisler found that dim artificial lighting could adjust the circadian rhythm between 22-28 hours. This means the results may lack validity as the sleep/wake cycle may vary considerably more than suggested
• In his work, Siffre was the only participant. Siffre noted that when he went into the cave at 60 years old, his body responded very differently, with his body following more of a 48 hour cycle. In contrast, Czeisler has found evidence that cycles vary by as much as 13-65 hours. This means the results may lack generalisability to other individuals and populations since individual differences may affect our rhythms and these haven’t been accounted for
• Aschoff and Weber (1962) studied participants in a bunker that had only electric light and no windows. The ps could turn the lights on and off as they wanted, so the light source fitted with their body clocks. Eventually, their body clocks settled into a sleep/wake cycle of 25-27 hours
• Folkard (1985) they withdrew ps from natural light for three weeks. Without the ps’ knowledge, time cues were changed to 22 hours a day. Only one participant out of twelve couldn’t adjust to the shortened day