Serving as the framework for physical and mental functioning, adequate sleep is essential for the human brain and body for recovery, learning, performance, and cognition. However, many experience fluctuating sleep patterns and sleep deprivation. Often insistent upon is that sleep is determined by solely internal processes such as the level of stress, amount of certain chemicals, or the intensity of tiredness. Granted that these can affect sleep, external sources such as light have an even greater role in regulating when and how well you sleep.
Stanford School of Medicine Professor Andrew Huberman describes how sleep works in the brain and body, generating an actionable solution to control a fundamental variable that governs when you fall asleep.
Sleep is determined by two factors: the buildup of adenosine and the circadian rhythm. Adenosine is a molecule that drives sleepiness and tiredness, building up the more you are awake. This means that in the morning, after restful sleep, your adenosine levels are low, while in the evening, after being awake for 12-16 hours, your adenosine levels will be high and you will feel sleepy.
Though adenosine builds up, if you pull an all-nighter, you’ll notice that as morning comes, you’ll notice an increase in energy, this is because of the circadian rhythm.
What is the circadian rhythm?
Existing in the brain of every single animal, the circadian rhythm refers to a 24-hour clock that starts the functions of all of our body systems and determines when we want to be sleepy/awake.
Our sleep is condensed into one 6-10 hour block. (The circadian rhythm makes it so that we’re not getting sleepy every thirty minutes to an hour, but rather makes us sleep during a designated time).
Cortisol, Epinephrine, and Melatonin
Waking up is caused by a hormone called cortisol (involved in stress and alertness) that is released from the adrenal glands. A hormone is a chemical substance released from an organ in your body that acts on other organs.
Adrenaline and epinephrine are also released from the adrenals and brain. When this cortisol pulse takes off, you feel more alert and awake, but this also sets a time in your body and nervous system that determines when melatonin will be released from your brain.
“The wakefulness signal triggers the onset of the timer for the sleepiness signal”– Huberman
Once awake from the cortisol release, a signal is sent that in 12-14 hours melatonin, the molecule that lets you fall asleep, will be released from your pineal gland, the only region in your brain that naturally secretes melatonin.
The cortisol and melatonin rhythms are endogenous — regardless of any external input (including light) the rhythm would still continue.
Setting the circadian rhythm
While food intake, exercise, and showers can affect sleep, the determining factor in setting and adjusting your circadian rhythm is light exposure. Light is so crucial in setting the circadian rhythm that primitive eyes in animals could only perceive light because it is the only way light gets into the body. Then the secondary useful characteristics of vision developed only after the ability to perceive light.
Light, mRGCs, and the super chiasmatic nucleus
A set of neurons in your eye — melanopsin retinal ganglion cells(mRGCs) — perceive particular kinds of light that trigger the start of your circadian rhythm. These cells are located in the bottom half of your retina, where light can reach them directly from overhead.
When light enters the eyes, photosensitive ganglion cells, mRGCs, collect the light and trigger for the master clock, the super chiasmatic nucleus, to set the circadian rhythm.
The super chiasmatic nucleus resides in a region right above the roof of your mouth and connects with every cell and organ in the body. It sets the circadian rhythm in two ways: (1) by secreting a peptide into the bloodstream that travels around the body and ‘tunes’ the cells to start the clock/circadian rhythm; (2) internal temperatures rise.
Your brain adjusts to these changes every few days with short-term neuroplasticity, or learning through rewiring connections between neurons. Because the mRGCs and the super chiasmatic nucleus change neuroconnection quickly, it is important to view light on a regular basis for optimal sleep, waking up and falling asleep at roughly the same time each day.
A protocol for optimal circadian rhythm adjustment
For maximal results, the best times to view the sun are in the morning, within 1-2 hours of the sunrise and during the evening, within 1-2 hours of the sunset. These times can be slightly variable; however, the significance of viewing the sun at this time versus when it is overhead in the afternoon is that the low solar angle produces a unique color and wavelength of light that the mRGCs are attuned to viewing.
Furthermore, it is critical to view the light directly, not through a window. This is because the glass of the window can scatter the solar wavelengths, reducing the intensity of light entering the eye to less than 50 times. Nevertheless, wearing corrective glasses does not detract from the intensity of light because the lenses in glasses are designed to concentrate the wavelengths of light directly onto the retina; therefore the light is not scattered and glasses do not act as a barrier to viewing light.
Light intensity varies in different weather conditions, seasons, or regions of the world relative to the equator. On a bright sunny day, it is sufficient to view the sun for 2-3 minutes to activate the circadian rhythm. Cloudy days with low solar intensity require for a longer time — 5-10 minutes. This is because it is still really bright outside on a cloudy day compared with a lamp or any source other than the sun, even if it is really, really bright.
While viewing light can aid in sleep, viewing it at the wrong time can hinder how well you sleep. The sleepier you get, the more light-sensitive you become, so later in the evening it is much easier to trigger wakefulness via light, even if the light being viewed isn’t particularly bright. This is particularly related to blue light in computer and phone screens and overhead lights. Even if your phone is in low brightness, the photosensitivity of your eyes in the evening is able to detect even that low light, signaling wakefulness. Overhead lights also stimulate wakefulness more than lamps or light-emitting objects toward the ground because the mRGCs are located in the bottom half of the eye looking overhead; therefore, if the light is on the ground it does not directly shine on the mRGCs.
