The Mechanism of circadian rhythm - Explaining Why You Feel Drowsy at Night
Hi everyone, I hope you are having a wonderful day. Today, I will be discussing the mechanism of the suprachiasmatic nucleus which is basically how it affects the release of Melatonin. Have you thought of active you can be during the day, doing multiple jobs and working from clock in to clock out? Also, at night you can be very active when there is a bright light, only for you to start feeling drowsy and sleeping when the lights are dim or off. The human body has its own way to regulate itself to rest when it needs it, being able to identify night and day, when to be active, and when to be relaxed. In this post, we will look into how the body regulates itself to be able to identify how the body does this, what is responsible for this activity, and how to understand this activity better.
Before we continue, it will be good that we explain a few jargons here. What is the suprachiasmatic nucleus and where is it located, also what is melatonin and where is it located? Very quickly, the suprachiasmatic nucleus is a cell located in the hypothalamus, in the brain. It is referred to as the body's biological clock which tells the body the time to sleep or the time to wake. It is responsible for controlling the circadian rhythm which changes physical, mental, and behavior in a 24-hour cycle. Melatonin on the other hand is a hormone from the pineal gland which is located in the brain. Melatonin is the hormone that regulates the circadian rhythm which is responsible for inducing feelings of sleep drowsiness, body temperature reduction, and sleep.
When light strikes the retina, there are specific cells different from the normal photoreceptor cells in the eye, they are referred to as intrinsically photosensitive retinal ganglion cells which are sensitive to light and their function is to transmit the information on the amount of light entering into the eyes, to the brain. The information is transmitted to the brain through the retinohypothalamic tract. The retinohypothalamic tract sends the information to the suprachiasmatic nucleus. When they receive information of light, they are activated and glutamate is released. The glutamate from the suprachiasmatic nucleus then activates the dorsomedial hypothalamic nucleus. The activation of the dorsomedial hypothalamic nucleus leads to the activation of the Locus Coeruleus through a neurotransmitter known as Orexin. The Lucus Coerulus then activates the intermediolateral nucleus through norepinephrine noradrenaline through the alpha 1 adrenergic receptor on the intermediolateral nucleus. The intermediolateral nucleus in the presence of light releases acetylcholine which then inhibits the superior cervical ganglion (SCG). While the superior cervical ganglion is inhibited in the presence of light, at night it is active and stimulates the pineal gland through norepinephrine to secrete melatonin into the blood. This activation is done through a Beta-1 adrenergic mechanism. During the day, the intermediolateral nucleus inhibits the superior cervical ganglion thereby preventing the release of norepinephrine. This happens when there is total darkness but it is important to know that at night, in the presence of light, but when there is no light, the intrinsically photosensitive retinal ganglion cells are not activated thereby inhibiting the suprachiasmatic nucleus, which inhibits the dorsomedial hypothalamic, inhibiting the intermediolateral nucleus, causing the superior cervical ganglion to be activated which in turn activates the Pineal gland to release Melatonin which is the sleep hormone.
Also, when there is light, the intrinsically photosensitive retinal ganglion cells recieves the light, it sends the information through the retinohypothalamic tract to reach the hypothalamus where the suprachiasmatic nucleus activated to release GABA which is an (inhibitory central nervous system neurotransmitter) to the paraventricular nucleus thereby inhibiting the paraventricular nucleus. The paraventricular nucleus is responsible for inhibiting the intermediolateral nucleus in a dark environment by releasing vasopressin to the intermediolateral nucleus. In the presence of light, when the suprachiasmatic nucleus inhibits the paraventricular nucleus, it will cause the intermediolateral nucleus to be activated causing the inhibition of the superior cervical ganglion thereby preventing the release of norepinephrine to the pineal gland, thereby preventing melatonin release. When there is no light, the suprachiasmatic nucleus is not activated which in the end doesn't release melatonin.
As a traveler, you might be able to travel from one country to another without shutting your eyes all through the night, or maybe you work all night or stayed awake for about 2 days and nights, and you start to feel sick, confused, agitated, and so on. This is because the body has a biological system that syncs with the normal day and night, we could call the body clock, but biologically, it is referred to as the Circadian rhythm. If there were no technology and innovations which birthed the bulb and electricity to give us light at night, the body's Circadian rhythm which is synced to the days sunrise and the night sunset would have been doing its job on a regular, anyways, it still does but a lot of us can now stay for days without sleep since we have light reflecting into our retina. Even though we have lights at night now, messing with the Circadian rhythm of the body can affect sleeping, eating, and mating, in such a case that testosterone secretion, bowel movement suppression, and so on are affected.
It is important to know that the Circadian rhythm is controlled by the hypothalamus which is at the base of the brain. The Circadian rhythm is triggered by the suprachiasmatic nucleus which is connected to the retina. In the presence of light, our heart rate, blood pressure, temperature, and other activities are increased while melatonin is inhibited and when there is no light, temperatures, heart rate, blood pressure, and so on reduce, and melatonin is secreted into the blood.