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The sleep-wake cycle: What happens when you sleep?

We spend one-third of our life sleeping, which equates to 9,490 days or 227,760 hours! Undoubtedly, sleep is one of the most important parts of our day. Sometimes you drift off to sleep easily after a tired day, but other times you toss and turn for hours before you manage to fall asleep. After lunch, you may be yawning, wishing to be in bed. Near midnight, you might be wide awake with loads of energy! This can all be explained by the sleep-wake cycle, which is a circadian rhythm.


All living organisms are subjected to biological rhythms, but psychologists are especially interested in studying the sleep-wake cycle. This is because the quality of sleep could greatly influence our behaviour.


All biological rhythms are controlled by two things – your internal body clock, known as the endogenous pacemaker, and external changes in the environment, known as exogenous zeitgebers. Circadian rhythms are those rhythms that last for around 24 hours (circa means ‘about’ and diem means ‘day' in Latin).


The fact that we feel drowsy when it's night-time and energised during the day demonstrates the effect of daylight - an important exogenous zeitgeber - on our sleep-wake cycle. However, the sleep-wake cycle is also governed by our endogenous pacemaker - a biological 'clock' called the suprachiasmatic nucleus (SCN). The SCN lies just above the optic chiasm which provides information from the eye about light.


The sleep-wake cycle may also explain one common phenomenon that we all have experienced – jetlag. For example, if you leave London on a flight at 4 p.m. on Tuesday and arrive in Hong Kong at 12 p.m. Wednesday, your internal clock still thinks it’s 5 a.m. This is because the sleep-wake cycle is a ‘free-running’ cycle, which means your body will fall asleep and wake up on a regular schedule. With entraining by an exogenous zeitgeber (e.g. light) and a little bit of time, this could help the body adjust to your local time zone.


Useful research done on the sleep-wake cycle is benefitting us as students! Wolfson and Carskadon (1998) recommend that the school day start a couple of hours later to fit in with the typical teenage sleep pattern. Hormonal shifts in the teenage body mean that getting to sleep becomes more difficult than adults, and therefore teenage students tend to be tired and sleepy at the start of the school day. Research has shown benefits for academic and behavioural performance when lessons start later in the day, including reduced dependence on caffeine (Adolescent Sleep Working Group 2014).


The next time you have a 9 a.m. class and feel exhausted, you can present your knowledge on the sleep-wake cycle to your teacher as evidence to convince them that it's not you, it’s your sleep-wake cycle!

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