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Medically Reviewed by Dr. Joshua Tal
What are circadian rhythms, and how do they affect your daily sleep and wake patterns? Simply put, nearly every biological process is governed by a rhythm or pattern. The word circadian was coined by biologists in the 20th Century, from circa, the Latin term for around, and diem for day. Other terms for biological rhythms are ultradian, which are short-term cycles, and infradian, which are longer than a day.
Ultradian rhythms include eye blinking, heart beats, and breathing. Infradian rhythms are more evident in animals; the most prominent example in humans is the menstrual cycle. These daily rhythms have been found in living beings ranging from unicellular bacteria to higher mammals. While most animals and plants have cycles tied to the sun, some plants and animals in tidal regions have cycles that also correlate with the moon and tides. The circadian rhythm takes into account both the sun and the moon, using the moon’s gravitational orbit around the earth, sunlight, and the earth’s resulting 24 hour rotation.
At the organism level, a master circadian oscillator in the brain’s hypothalamic suprachiasmatic nucleus (SCN) orchestrates tissue-specific rhythmic activities. The SCN, located in the brain’s hypothalamus, is a small area with a big influence. Discovered in 1972, it functions as a master clock for the body. The SCN receives information directly from ganglion cells in the retina of the eye, turning light and darkness into information indicating the release of hormones to synchronize the body’s functions to the 24 hour cycle. The SCN uses the pineal gland to relase the hormones, primarily cortisol and melatonin, with cortisol stimulating wakefulness in the morning and melatonin decreasing wakefulness at nighttime.
Scientists once believed that teenagers and young adults had a slightly longer circadian clock, with 25 hours in each day. This was thought to explain why older adults slept less than younger adults. However, researchers now state that older adults do not sleep less, but rather are more prone to earlier and desynchronized circadian rhythms, such as going to bed earlier and waking up earlier, or napping more and developing irregular sleep routines. This may be due to the lack of structured daily activities that come with retirement, as well as age-related differences in sleep patterns to neurological changes that begin in midlife.
For example, research shows a decline in slow-wave sleep and an increase in fragmented sleep as we age. The brain’s production of growth hormone and melatonin also decrease, causing sleep cycles to shift for seniors. This may be why melatonin supplements and melatonin agonists are shown to primarily work as a sleep aid for older adults and not for younger populations. It’s also thought that retirement may play a role in disrupted sleep patterns, because retirement removes external social circadian cues, like a daily alarm clock. Age-related declines in vision may also play a role, as cataracts reduce light signals hitting the retina, an important circadian cue.
To help maintain healthy circadian patterns, seniors can seek out intentional exposure to zeitgebers, a German word that means time signals. These include light, temperature, ambient noise and meal times. Seniors can increase exposure to zeitgebers by taking a morning walk, meeting with a social group at a regular time, and keeping meal times and sleep times consistent.
Light is one of the most powerful factors regulating the body’s circadian rhythm, and seasonal shifts to sunrise and sunset can impact sleep patterns. We tend to sleep less and feel less tired in the summer when daylight is at its peak, and feel more tired during winter’s darkest days.
Scientists have observed seasonal changes in melatonin production, and resulting shifts in behavior, in humans and animals, though it isn’t yet known how these changes may impact biological processes. For some animals, circadian cycles dictate melatonin release, which in turn cues seasonally influenced behavior like migration, mating, and molting.
To a certain extent, your own circadian patterns may be influenced by your genes. Biologists have identified genes that predispose you to a certain schedule. Some people are more disposed to a circadian delay – falling asleep later and waking up later – and some people are more disposed to a circadian advancement – falling asleep earlier and waking up earlier. Just because you have a disposition towards one or the other does not mean it will necessarily be expressed that way. There are some established methods for changing your circadian rhythm using practice and consistency, melatonin, or light therapy. In fact, scientists have discovered that setting a consistent wake up time – both weekdays and weekend – can be a powerful method for shifting your circadian rhythm to more desirable timing.
Scientists have determined that the molecular timing mechanism (a.k.a. biological clock) works at the cellular level. Even cells in a petri dish have a time cycle. Inside the cell, proteins are transcribed and translated in a feedback loop. This cycle in turn regulates transcription of a large number of genes.
Misalignment of the circadian rhythm can negatively impact physical and mental health; circadian rhythm disruption is linked to a number of health problems including obesity, depression, Seasonal Affective Disorder (SAD), and cardiovascular disease. Sleep and Circadian Rhythm Disruption (SCRD) is a condition in which the body’s natural circadian patterns don’t function properly, resulting in disrupted sleep patterns. SCRD is linked to mental illness, suggesting the vital importance of circadian patterns to both physical and psychological wellbeing.
Maintaining a regular sleep routine and consistent sleep and wake times is one of the most effective ways to support a healthy circadian cycle. Other strategies include: