Sleep Wake Cycle

How do biological mechanisms regulate the sleep-wake cycle?

There are internal biological mechanisms responsible for our sleep cycles: circadian rhythms and homeostasis. When people are awake for a long time, the homeostasis sleep pressure sends negative feedback to the body—signalling the need for sleep to reset the energy levels.

Homeostasis maintains the internal bodily environment in constant equilibrium through a series of dynamic interactions in physiological and biochemical systems. The homeostasis mechanism and the circadian rhythms, one of the biological rhythms, work together to determine the sleep-wake cycle.

Circadian rhythms control many of our daily functions, for example, by shifting our physiological temperature and controlling our cognition, core body temperature, metabolism and even hormone production and appetite. They also regulate our sleep-wake cycle to keep a balanced level of energy.

Have you ever wondered why, on certain weekends do you wake up earlier than you wish? Or what about when you forget to set the alarm clock, but it happens that you wake up just on time! That can occur due to your body’s biological clock (endogenous pacemaker), based on a schedule of around 24-hours (one day) and controls most circadian rhythms. Circadian rhythms are synchronised with the natural environment (exogenous zeitgeber), but the changes will continue occurring even if the person is isolated from the external environment.

The homeostasis drive increases throughout the day. Usually late in the evening, when most people need to sleep, it reaches maximum capacity. Physical exhaustion, mental work, staying up for long hours, or a low immune system can increase sleep pressure. You can achieve a healthy sleep-wake cycle when the circadian rhythms and homeostatic sleep pressure are synchronised.

Which hormones are involved in the sleep-wake cycle?

The most important exogenous zeitgebers or external influence affecting the internal biological clock is the sunlight. When the SCN senses light through the retina of the eye, it releases chemical signals. Cortisol and neurotransmitters, like norepinephrine and serotonin, are released to keep the brain awake and alert.

As the day progresses, a chemical called adenosine accumulates in the bloodstream, signalling the feeling of tiredness. As the daylight shifts to the dark, melatonin levels rise, making you feel sleepy. During sleep, while melatonin hormones rise, adenosine levels decline.

Once the brain is asleep, a cleaning process starts. The glymphatic system clears out proteins present in the brain. The glymphatic system increases its activity during sleep. It could explain why sleep helps heal the body after strokes and traumatic brain injuries. However, more research is needed to support the studies; nowadays, doctors are studying how the glymphatic system relates to dementia and Parkinson’s.

How is melatonin involved in the sleep-wake cycle?

The pineal gland in your brain produces melatonin, which controls your sleep-wake cycle. Your internal biological clock influences the quantity of melatonin produced according to exposure to sunlight. Usually, melatonin levels are more elevated during the mid-to-late evening, after sunset. However, when the sun is rising, the levels drop, supporting the awakening process.

The melatonin hormones are responsible for keeping the circadian rhythm flowing in harmony. When the retina detects light, melatonin production starts to decrease, and wakefulness starts; light wavelength (colour) and intensity are important factors that affect melatonin production. On the other hand, in the absence of light or darkness, the production of melatonin increases, and the body starts preparing for sleep. Changes such as decreases in body temperature and blood pressure are generated when melatonin inhibits the SCN.

The light activates photoreceptor cells in the retina, which signal the SCN via neurons of the retinohypothalamic tract.

How is serotonin involved in the sleep-wake cycle?

Serotonin is released to signal to the body that it needs to sleep or ‘homeostatic sleep pressure’. Circadian rhythms and homeostatic sleep pressure are necessary for a healthy sleep-wake cycle.

How is cortisol involved in the sleep-wake cycle?

Cortisol keeps the maintenance of the sleep-wake cycle, as it stimulates wakefulness in the morning and by continuing to support alertness as the day goes by. It is progressive in allowing the internal body rhythms, sleep drive and other chemicals, such as adenosine and melatonin.

Elevated levels of cortisol may be the reason for sleep disruptions such as insomnia and sleep apnea as the 24-hour rhythm is disrupted. It contributes to health problems that collaborate with unhealthy sleep patterns. When cortisol levels are frequently high, and over long periods, it causes health issues; they might be chronic illnesses (high blood pressure, diabetes and heart disease) such as:

• Weight gain.

• Fatigue.

• Issues with memory and focus.

• Increased inflammation and vulnerability to disease.

• Increase ageing.

• Digestive problems.

• Mood disorders, like depression and anxiety sleep-wake issues.

How is histamine involved in the sleep-wake cycle?

The histamine hormones are also very important when talking about the sleep-wake cycle(localised within the hypothalamus), it is released in areas of the central nervous system.

Histamine release is highest during wakefulness. The histaminergic neurons have maximal activity during high vigilance and cease their activity during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep.

How can you improve your sleep-wake cycle?

You can improve the quality of your sleep-wake cycle by incorporating healthier habits, which can improve your health. If you have insomnia or have any other sleep difficulties, you can follow the suggestions below. However, we suggest you look for medical support, as certain circadian rhythm disorders cannot be managed without medical help.

• Have a consistent sleep-wake cycle.

• Expose yourself to sunlight during the daytime.

• Eat healthily and exercise regularly.

• Avoid digital devices such as mobile phones that emit blue light waves before bedtime.

• Be aware of your caffeine intake times.

• Keep the bedroom dark, cool, and quiet.