Section Summary

Endogenous biological rhythms are found in nearly every organism on the planet. Internal clocks can help an organism anticipate changes in their environment. There are many types of biological rhythms that can range from seconds to hours to years; circadian rhythms are those that take about 24 hours to complete a cycle. We can characterize the components of a rhythm by analyzing their period, phase, amplitude, mesor and frequency. Furthermore, we can determine how an internal clock responds to the environment by constructing a phase response curve.

Your brain contains a master clock which regulates your daily rhythms from your gene and protein changes in cells to your sleep patterns. In mammals, this clock is found in the SCN, a collection of cells that express clock genes, neurotransmitters and neuropeptides. Elegant lesion and replacement experiments established the SCN as the site of the master clock. The retina sends information to the SCN via a number of connecting pathways, including ipRGCs that contain melanopsin. The SCN also expresses clock genes that have a feedback loop that regulates daily rhythms. There is a diversity of species that exhibit biological rhythms, and thus additional research is needed to determine when, how, and why these rhythmic processes emerged.

There are several overlapping hypotheses for why sleep exists including energy conservation, restoration of energy, and aiding growth and development. Sleep can be measured in the lab using a PSG which detects changes in the phases of sleep. There are non-REM (3 stages) and REM sleep stages that are differentiated by different types of brain wave activity patterns. The flip flop model of sleep-promoting neurons in the VLPO have a mutual inhibition with wake promoting cell populations in the brainstem and hypothalamus. This is referred to as a flip flop switch and is maintained in part by a variety of neurotransmitters including acetylcholine, norepinephrine, serotonin and GABA. Finally, there are sex differences in sleep that may be influenced in part by circulating hormones.

Disorders of sleep and circadian rhythms can have a serious impact on the lives of those suffering from such conditions. Such conditions are common but often go untreated. Examples discussed in more detail are non-24 hour sleep/wake disorder, narcolepsy, and delayed sleep-wake phase syndrome.

We can become sleep deprived due to shift work, daylight savings time changes, or reducing our sleep to gain study time. Sleep deprivation and disruptions to the circadian system can have a major negative impact on human health, but treatment of many conditions is effective through behavioral interventions. Furthermore, relief of circadian and sleep symptoms can contribute to improvement in underlying comorbid conditions.