Circadian Rhythms and Zeitgebers

brown brain decor in selective-focus photography

How External Cues Regulate the Body Clock

The vast majority of lifeforms on Earth are subject to the influence of daily biological rhythms, from sunflowers moving their massive heads to follow the sun across the sky, to laboratory fruit flies altering activity levels in response to temperature changes. Humans are no exception to this general trend, and normally adapt to external time cues, known as zeitgebers, by altering their normal periodic fluctuations in hormones, brain wave patterns, and body temperature.

Biological clocks work on predictable schedules that follow natural patterns on a daily, monthly, or annual cycle. The Earth’s orbit around the sun, its rotation on its own axis, and the influences of moon cycles tell birds when to migrate, flowers when to bloom, and humans when to eat, sleep, and reproduce. The fluctuations that occur on a daily basis, known as circadian rhythms, are largely governed by the influences of light and temperature fluctuations over the course of the day.

In today’s decidedly un-natural environment, the natural rhythms of sleeping and waking can be altered by 24-hour daylight and erratic work schedules. Working with the body’s natural reactions to external stimuli is a simple way to improve sleep quality and normalize circadian rhythms.

Circadian Rhythms in the Absence of Zeitgebers

According to Essential Psychopathology and its Treatment (Maxmen and Ward 1995)when humans are secluded in deep caves and isolated from external zeitgebers, their body clocks are forced to regulate based on internal neurological mechanisms. If an average person is placed in an environment devoid of zeitgebers, their biological clock increases from a 24-hour schedule to a 26-32 hour cycle.

This varies based on age as well as individual predisposition. Generally speaking, teenagers tend to have a much longer natural biological clock than adults. Because of this, they tend to perceive time as being earlier than it actually is. This explains, in part, why teens are so prone to staying up until all hours and sleeping late. Older adults, by contrast, have a shorter natural cycle that causes people to fall asleep earlier as they get older.

How Zeitgebers Influence the Body Clock

The shifts in day length with changing seasons naturally tell organisms when is the optimum time for biological activities such as sleeping and waking. It is not surprising, then, that according to the authors of Clocks and Rhythms (Stillman et al 2008,) the most powerful zeitgebers for most organisms are temperature and light.

Circadian rhythms are regulated by a number of neurological mechanisms and structures, chief among them the suprachiasmatic nucleus. According to Psychology (Santrock and Mitterer 2008) this small structure within the hypothalamus uses light cues obtained from the retina to regulate its own rhythm. It can then, in turn, influence other neural structures to stimulate biological activities such as sleep, wakefulness, hunger and body temperature in a way that works with environmental conditions.

In the modern world, other zeitgebers can override natural seasonal changes in day length. Some man-made influences on circadian rhythms include:

  • artificial light
  • externally regulated sleep patterns, such as shift work schedules
  • clocks, radios, and televisions
  • global travel

These factors can disrupt the body’s natural cycles, leading to disturbed sleep and delayed-onset sleep phase. In the absence of regular, predictable patterns of light and darkness, the structures of the brain may have difficulty reconciling the natural, internal rhythms with those of the outside world.

Using Zeitgebers to Regulate Sleep

For an average person, sleep cycles can often be regulated simply by carefully manipulating common zeitgebers. Falling asleep and waking up at the same time every day, and controlling the light and temperature of the sleeping area on a regular schedule will program the brain to release sleep-inducing hormones at the appropriate time.

By regulating the body clock, individuals can naturally stimulate sleep onset at a regular and predictable time. This is a natural, healthy, and drug-free way to ensure a good night’s sleep and an alert, well-rested morning.


Maxmen, Jerrold and Nicholas Ward. Essential Psychopathology and its Treatment. New York: W.W. Norton and Co.

Santrock, John and John Mitterer. Psychology. Toronto: McGraw-Hill Ryerson.

Stillman, Bruce, Terri Grodzicker and Cold Spring Harbor Laboratory . Clocks and Rhythms. Cold Spring Harbor: CSHL Press.