We all have a circadian clock that regulates the functioning of our bodies through the interplay of two hormones: cortisol (wakefulness) and melatonin (sleepiness). This is true even if we are subjected to total darkness. With one catch. Experiments have shown that in the absence of light, our circadian clocks start drifting later and later. That’s because sunlight works as an anchoring mechanism for this rhythm.
According to Dr. Andrew Huberman who leads a brain research lab at Stanford, we have a type of neuron in the back of our eyes called retinal ganglion cells. Among other things, these neurons perceive the specific type of light being received by the eye and send electrical signals to the region of the brain responsible for controlling circadian rhythms (the suprachiasmatic nucleus). One way to get your cortisol and melatonin rhythms cycling properly is to get direct exposure to both early morning and sunset light.
Retinal ganglion cells respond optimally to the particular quality (i.e.contrast of yellows and blues) and intensity of light we get from the sun at the beginning and end of the day. Ideally, we can get that light exposure directly without the filtering that happens through windows or eyeglasses. According to Huberman 2-10 minutes of exposure should suffice for most latitudes.
Go deeper: listen to Hubberman’s podcast episode “Master Your Sleep & Be More Alert When Awake.” If it is difficult for you to get outside in the early morning or around sunset, you can try this sunrise/sunset simulator. It’s highly rated by both Wirecutter and Wired.