Sleep - the resting state in which the body is not active and the mind is unconscious (Cambridge dictionary)
Every day, most of us sleep, for an average of around 8 hours a day, meaning that ⅓ of our life is in sleep. We can pull “all-nighters” but not for long; sleep is one of the most regular and co-ordinated cycles in humans. Then, how exactly does this period of rest work, and why is it so pertinent to our functioning as human beings?
Interestingly, in Latin, “circa” means approximately, and “dian” refers to the goddess of the moon, diana. The “circadian rhythm”, perhaps refers to a rhythm “controlled” by the goddess of the moon.
The circadian rhythm
The Circadian Rhythm, also known as the sleep-wake cycle, is an endogenous rhythm (originating from within the body) that repeats every 24 hours. It induces physical, mental, and behavioral changes, affecting most living things, including humans. This pattern is thought to be mainly controlled by hypothalamus (near the pituitary gland), specifically the suprachiasmatic nuclei, which is situated above the optic chiasm. It stimulates the production of many neuronal and hormonal activities. The neurons in the suprachiasmatic nuclei are unique in that they have many synapses between the dendrites.
The suprachiasmatic nuclei control the production of melatonin, a hormone that induces sleepiness within us. The level of melatonin produced is based on the ambient light around us; this is relayed through the optic nerves (connects the eye to the brain). When there is less light, more melatonin is produced, and vice versa.
Stages of Sleep
We can see the brain’s electrical activity through electroencephalography. Even when we are asleep, our brain still shows a multitude of activity. When conscious, the brain exhibits low amplitude electrical activity.
Throughout the night, our brain cycles through the four main stages of sleep - one for rapid eye movement and the other three for not rapid eye movement sleep.
When initially falling asleep, the electrical activity of the brain flattens, but gradually increases in amplitude and decreases in frequency (it is a wave!) as one moves through different sleep stages, known as slow-wave sleep. It is unknown why these changes occur; however, one prevailing theory is that certain brain neurons stop responding to their typical inputs (such as the ones controlling the arm or the leg). These neurons are inhibited.
Stage 1: the initial ‘falling asleep”; typically lasts 1 - 5 minutes. The body and brain activities begin to slow, but the body isn’t fully in a state of relaxation. Consequently it is easier to wake someone up
Stage 2: the body enters an increased state of relaxation; typically lasts for 10 - 25 minutes. It drops in temperature, the muscles relax, and both the heart rate and breathing rate relax. Eye movement stops. There are short bursts of activity seen in the electrical activity of the brain - this perhaps can help one resist being woken up
Stage 3 (deep sleep): it is more difficult to arouse someone from their slumbers in this stage. This stage typically lasts for 20 - 40 minutes each. Muscle tension, heart rate, and breathing rate all decrease, allowing the body to further relax. Many experts theorize that this stage is “restorative”, bolstering growth, the immune system, healing, etc; there is evidence of it fostering critical thinking, memory, and creativity
Stage 4 (Rapid eye movement): brain activity has nearly increased to the amount when one is awake. In this stage, in order to prevent one from moving around, the muscles are paralysed (atonia), excluding the breathing muscles and eye muscles. (Otherwise, we would choke!). Most dreams occur during REM sleep, and during REM dreams are more vivid.
We hope that this short intro to sleep has taught you more about this fascinating activity every single one of us do. Take care of yourself and sleep more :)