Circadian rhythms refer to regular or rhythmic variations in biological and behavioral measures approximately (“circa”) on a 24-hour or daily (“dian”) timing basis. Virtually all biological and behavioral measures taken over time show circadian rhythms. For example, core body temperature normally reaches its highest value in the early evening and lowest value between 4 and 6 a.m.
Our biological circadian rhythms strongly determine our level of alertness and ability to sleep. At our maximum core body temperature, our alertness is greatest. At the minimum body temperature, alertness is diminished and sleepiness is maximum. Night shift workers experience these effects when they have difficulty maintaining alertness on the job at around 4 to 6 a.m., but then have difficulty sleeping during the day. Another example is the difficulty with nighttime sleep and daytime alertness following transmeridian jet travel, known as jet lag.
Our circadian rhythms can drift later or earlier and cause difficulties sleeping at our chosen times. Evening types or “night owls” have difficulty getting to sleep before midnight and difficulty arising early. They will lose sleep if they have to stick to a consistent early wake-up time. Sleep-onset insomnia is associated with late timed circadian rhythms. Morning types or “larks,” in contrast, struggle to stay awake in the evening but then have difficulty maintaining sleep. This can lead to early morning awakening insomnia caused by circadian rhythms timed abnormally early. If the difference between maximum and minimum values in a rhythm (amplitude) is reduced, it can impair daytime functioning and disturb nighttime sleep.
During postnatal development, there is a progressive shift from a 4-hour sleep/wake rhythm toward increasing amplitude circadian rhythms and the consolidation of sleep into the night period. This consolidation is usually well progressed by the age of 6 months and complete by early school years. However, the broken sleep of infants can disrupt parental sleep and be stressful to a family. A common circadian rhythm difficulty in adolescence is a delayed rhythm in which the early evening alert period is delayed, making it difficult to initiate sleep before midnight. Awakening for school at 7 a.m. is doubly difficult due to insufficient sleep and the maximum sleepiness associated with the low body temperature occurring at that time. Attempts to catch up on lost sleep on the weekend by sleeping in late only further delay the circadian rhythms and exacerbate the problem. The middle-aged and older populations more commonly have the opposite problem of early timed circadian rhythms and early morning awakening insomnia. The elderly in nursing homes often have reduced amplitudes of circadian rhythms associated with reduced daytime activity and very disturbed nocturnal sleep.
Circadian rhythms can be retimed and amplitudes enhanced. The best known tool for retiming the rhythms is bright light visual stimulation, particularly light from the blue end of the color spectrum, which can be provided by daylight. In infants and the very elderly, amplitudes of circadian rhythms can be enhanced and timing of rhythms stabilized with increased daylight exposure. The delayed rhythm problems of adolescents can be treated with consistent early morning daylight exposure. The early morning awakening problems in older people can be treated with evening bright light and avoidance of early morning daylight. Where daylight is insufficient in the winter, especially in northern latitudes, portable light devices have been developed to be of therapeutic benefit.
- Lack, C., & Bootzin, R. R. (2003). Circadian rhythm factors in insomnia and their treatment. In M. Perlis & K. Lichstein (Eds.), Treatment of sleep disorders: Principles and practice of behavioral sleep medicine. Hoboken, NJ: Wiley.
- University of Chicago, Division of Biological Sciences— Sleep, Chronobiology and Neuroendocrinology Center, http://www.sleep.uchicago.edu/indehtml?content= studies.html
- Wright, R., & Lack, L. C. (2004). The effect of different wavelengths of light in changing the phase of the melatonin circadian rhythm. In S. R. Pandi-Perumal & D. P. Cardinali (Eds.), Melatonin: Biological basis of its function in health and disease. Georgetown, TX: Landes Bioscience. Retrieved from http://www.eurekah.com/abstract.php?chapid=1467&bookid=110&catid=48