All animals and plants have an inborn circadian clock that establishes a circadian rhythm of waking and sleeping. Hormones play an important and integral part of this daily dance from dawn until dusk.
The brain’s internal circadian clock – also known as the biological clock, body clock, circadian pacemaker, circadian system, and circadian oscillator – is centered in the hypothalamus region of the basal forebrain (low and in the front).
Each person’s circadian rhythm is determined or adjusted by external environmental cues called Zeitgebers, from a German word that means “time-givers.” The most important Zeitgeber is daylight.
The cerebral circadian clock naturally uses Zeitgebers to synchronize or reset itself every day to within just a few minutes of the Earth’s 24-hour rotation cycle.
Individual circadian periods vary, ranging between 23.5-24.5 hours in humans. A person’s PER (period gene) establishes the exact cyclical period with a mean average of around 24.2 hours – which happens to be just slightly more than the Earth’s rotational period.
About 1 in 4 people have a circadian period which is slightly less than the 24-hour day, while the remaining 75% have a circadian period slightly longer than 24 hours.
The brain’s circadian clock regulates sleeping and feeding patterns, hormone and urine production, alertness, core body temperature, brain wave activity, glucose and insulin levels, cell regeneration, and many other biological activities.
Hormones are chemical messengers in the bloodstream that circulate to all parts of our bodies. Their signals regulate the body’s growth and metabolic rates, sexual function, immune and reproductive systems, and many other chemical processes vital to good health.
Normally, our bodies produce these chemical messengers that transfer information between sets of cells to coordinate the proper functioning of all the body’s parts.
The word “hormone” comes from a Greek word (hormao) that means “I excite.” Hormones excite or stimulate a particular body part, called the target gland.
Collectively, all the glands in the body termed the endocrine system. Endocrinology is the medical study of hormones performed by specialized doctors (endocrinologists).
The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, reproductive organs (ovaries and testes), and pancreas.
The body manufactures hormones in the endocrine glands. From there, they hitch a ride, as it were, passing from within the gland’s cells directly into the blood flowing through the gland.
Two primary hormones related to sleep are affected by the circadian clock:
- Melatonin – produced in the pineal gland in the brain, chemically induces drowsiness and lowers body temperature
- Cortisol – produced in the adrenal gland, used to form glucose or blood sugar and to enable anti-stress and anti-inflammatory functions in the body
Humans are diurnal animals, meaning we are naturally active chiefly during the daytime hours, as mirrored by our circadian rhythms. Generally, the optimum part of the circadian cycle for the best sleep is when the core body temperature is at its minimum and melatonin concentration is at its maximum toward the end of the sleep period.
The human body functions at its best when its component parts follow consistent daily cycles that are in step with each other and with solar and social rhythms. Natural daily variations can aggravate pre-existing medical conditions in a time-dependent way.
For example, cardiovascular (heart) and cerebrovascular (brain) crises are most common in the hours after awakening when the circadian clock up-regulates cardiovascular activity as it prepares your body for a new day.
Some adults experience increased performance and productivity throughout the day after altering their work schedules to fit their chronotype – the optimum times to sleep and wake (morning or night). A night owl who likes to stay up late is likely to do well employed on a second-shift position whereas a lark who rises and shines with the dawn would thrive on the day shift.
Interval fasting is also trending now. Some wellness benefits have been reported by those who restrict their meals to the daytime or waking period of their circadian rhythms – say, 9am-5pm – and fast in between, drinking only calorie-less liquids such as water or tea.
A $24.5 million home in Beverly Hills, California, has raised the wellness bar by featuring a lighting system that can be set to mimic the sunrise and sunset to sync with the occupant’s internal circadian rhythm. This makes it easier to fall asleep and wake up. The bathrooms, in contrast, emit bright white light to boost morning energy levels.
Circadian rhythms may be adjusted by up to two hours or so either way according to an individual’s chronotype. In addition, hormonal rhythms adapt to environmental changes such as seasons of the year, the daily light-dark cycle, sleep, meals, and stress.
Aligning your daily activities with your native circadian clock can bring you peace of mind and body as your hormones do their jobs better – so don’t fight the feeling.