Synchrony of the circadian rhythm for healthy sleep

Biological rhythms are a fundamental element of our cells, organs and tissues. For human beings, just like all living organisms, rhythms and cyclic patterns are an integral part of our lives. The various cyclical patterns within our bodies, from organ and tissue activity, to hormone secretion, and genetic expression make the base of our day to day actions and behaviours. Of particular interest here, is the daily sleep-wake rhythm. As diurnal animals, humans are pre-programmed to be awake during the day and asleep during the night. Specifically, our behaviour is closely tied to our environment.

Light is known to play an integral role in maintaining synchrony of our biological clock not only to its environment, but across our different internal biological rhythms. Without proper exposure to the environmental light-dark cycle, our internal clock cannot properly regulate its internal rhythms.

Early cave studies have shown us that, without proper light exposure, our biological rhythms cannot remain synchronized and becomes ‘free running’. In one particular study in 198, two researchers, Kleitman and Richardson, isolated themselves in the Mammoth cave (Kentucky, USA) for a period of 32 days. The deepness of the caves made so that they were completely isolated from the environment’s time cues. Interestingly, throughout their time spent in the cave, Kleitman and Richardson’s daily rhythms and sleep patterns did not disappear but rather its duration changed and extended beyond a 24-hour cycle. These early findings provided two conclusions: (1) that we have an internal biological rhythm that dictates the timing of our activities, and (2) that light plays an important role in maintaining our rhythms aligned with the environment.

Since these early studies, further interest on the impact of our biological rhythms, not only on our sleep-wake patterns but our health and well-being, expanded into the field of Chronobiology.  Although there are a number for different biological rhythms, the more commonly known and of particular interest for the study of sleep is the circadian rhythm (circa, meaning approximately, and dian, meaning day). Daily rhythms include both physiological changes, such as the variability in our core body temperature and the production of melatonin during the night, and behavioural outputs, such as the sleep-wake cycle. 

With advances in research and the development of research protocols, including forced desynchrony and constant state studies, knowledge on the human circadian rhythm has expanded. In 1999, Czeisler and colleagues found that the average duration of the internal circadian rhythm is 24.2 hours. It is, however, important to note that there is a large variability between individuals for the length of their circadian rhythms. Whereas the majority of individuals have an internal circadian rhythm slightly greater than 24 hours (90% of the sample between 24 hours and 24.35 hours), some have an even greater rhythm or an internal rhythm that is slightly shorter than its environment.

As our internal rhythm is near but not quite aligned with the environmental 24-hour cycle, there is a constant need to adjust our internal clock to the environment. When deprived of environmental cues, the internal circadian rhythm runs at its own pace, and slowly loses synchrony with its environment. A more extreme version of this is experienced in situations of jet-lag, where our internal clock needs to adjust itself to the timing of its new environment.

As further discussed in Light Regulates our Internal Clock, light is the primary source synchronizing our internal clock to its environment. Proper exposure to light (preferably daylight) during the day-time and the absence of light in the evening and night is key to maintaining alignment of internal circadian rhythm to the environment 24-hour cycle.

For sleep, maintaining a synchrony to its environment has a particular role. Our internal circadian rhythm is one of the main drivers for both the timing and the quality of our sleep. Both biologically and socially, we are made to be awake during the daytime and asleep during the night. For more information, check out When Do We Sleep? When sleep is initiated out of phase with the circadian rhythm, or when the internal rhythm is not aligned with the environment, we experience a decrease in the quality and duration of sleep, including reduced time in REM sleep, and an increase in awakenings throughout the night. As a result, there is a decrease in both the quality and quantity of our sleep which, as we have all experience at one time or another, can have dramatic effects on our energy levels the following day.

However, the impact of a good and strong circadian rhythm expands far beyond being synchronized to its environment. The internal synchrony of our biological rhythms has been suggested to impact our health, including increasing risks of diabetes, obesity and even cancer. As such, with the important role of light on the maintenance and synchronization of our circadian clock and biological rhythms, proper light exposure both during the day and night is a critical factor not only for achieving good sleep, but can also play an important role in our overall health and well-being.

   Selected Supporting References:

  • Czeisler, C. A., Weitzman, E. D., Moore-Ede, M. C., et al. (1980). Human sleep: its duration and organization depend on its circadian phase. Science, 210(4475), 1264-7.
  • Dijk, D. J., & Czeisler, C. A. (1995). Contribution of the circadian pacemaker and the sleep homeostat to sleep propensity, sleep structure, electroencephalographic slow waves, and sleep spindle activity in humans. The Journal of Neuroscience, 15(5), 3526-38.
  • Czeisler, C. A., Zimmerman, J. C., Ronda, J. M., et al. (1979). Timing of REM sleep is coupled to the circadian rhythm of body temperature in man. Sleep, 2(3), 329-46.
  • Czeisler, C. A., Duffy, J. F., Shanahan, T. L., et al. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177-2181.
  • Czeisler, C. A., Zimmerman, J. C., Ronda, J. M., et al. (1979). Timing of REM sleep is coupled to the circadian rhythm of body temperature in man. Sleep, 2(3), 329-46.
  • Rudic, R. D., McNamara, P., Curtis, A. M., Boston, R. C., Panda, S., Hogenesch, J. B., et al. (2004). BMAL1 and CLOCK, two essential components of the circadian clock, are involved in glucose homeostasis. PLoS Biology, 2(11), e377.
  • Turner, P. L., Van Someren, E. J., & Mainster, M. A. (2010). The role of environmental light in sleep and health: Effects of ocular aging and cataract surgery. Sleep Medicine Reviews, 14(4), 269-280.
  • Sadacca, L. A., Lamia, K. A., Blum, B., & Weitz, C. J. (2011). An intrinsic circadian clock of the pancreas is required for normal insulin release and glucose homeostasis in mice. Diabetologia, 54(1), 120-124.

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