H567/03 Section B Option 3 - Biological rhythms and disruption
Explain biological rhythms, how internal timing is coordinated with the environment, and how disruption can affect sleep and behaviour.
Rhythm types and circadian timing
A biological rhythm is a recurring pattern in a biological process or behaviour. A circadian rhythm has a period of approximately 24 hours, such as daily changes in alertness, body temperature and melatonin secretion. An ultradian rhythm repeats more frequently than once per day, such as cycles within a night's sleep. An infradian rhythm has a longer period than a day, such as the menstrual cycle. Period means the time needed for a full cycle, not how strong the response is.
The daily sleep–wake pattern involves circadian timing, but sleep is also affected by how long someone has been awake and by demands such as caring for another person. A rhythm therefore does not force identical bedtimes or sleep durations in everyone. Chronotype describes differences in preferred timing: people vary in when they tend to feel most alert, and preferences can change with age and circumstances.
Circadian rhythms are endogenous, generated within the organism, rather than simply repeated reactions to a clock on the wall. However, they are normally synchronised with the environment. Without appropriate cues an internal rhythm can drift relative to clock time. Modern descriptions use a period near 24 hours; the older assumption of a universal 25-hour human cycle should not be presented as a current fact.
A fictional worker who is usually sleepy late at night and alert in the morning illustrates a daily timing pattern. A short nap does not instantly move that internal timing to a different time zone. The difference between timing and total hours of sleep becomes important when considering jetlag or rotating work.
Pacemakers and zeitgebers
An endogenous pacemaker helps generate and coordinate a rhythm. The suprachiasmatic nucleus (SCN) in the hypothalamus is a central circadian pacemaker. Light information from the eyes reaches this system, allowing internal timing to be aligned with the external light–dark cycle. Many tissues also have their own clocks; central coordination does not mean the body contains only one clock.
An exogenous zeitgeber, or time cue, helps synchronise internal timing with the environment. Light is especially important for circadian timing, while meals, activity and social schedules can also provide cues. Entrainment is the adjustment of a rhythm into alignment with such a cue. The internal rhythm is not abolished each night and recreated each morning.
The SCN helps regulate the timing of melatonin production by the pineal gland through downstream pathways. Melatonin normally rises during the biological evening and contributes to the body's night-time signal. The SCN is not the gland that makes melatonin. Artificial light during the usual biological night can interfere with timing, depending on its timing and other conditions; every exposure should not be assumed to have the same effect.
For example, a worker sleeps in daylight but encounters bright morning light when leaving a night shift. The light–dark pattern may continue to encourage daytime alertness and night-time sleep, while the job demands the reverse. This shows an interaction between an internal biological system and an external schedule. Neither biology alone nor the environment alone gives a complete account.
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