The Science of Sleep: Why do we get tired at night?

Have you ever gone to bed on Friday night, thinking that you had all the time in the world to sleep in, only to have woken up at your usual early hour? Maybe you have spent nights awake, unable to sleep despite feeling incredibly tired. Perhaps you don’t even need to set an alarm in the morning, and can naturally wake up at the same time every single day. These are all examples of the sleep-wake cycle in action, giving you signals about when to feel tired. 

The sleep-wake cycle exists as a part of one’s circadian rhythms, the 24-hour internal clock that keeps
track of all of your body’s functions. Circadian rhythms, from the Latin “circa diem”, or “around a day”,
help your body perform essential functions. These natural processes affect all living things, including animals, plants, and microbes. The internal clock, which dictates when it is time to go to sleep and when it is time to wake up, is made up of a group of over 20,000 neurons that join to create a structure in the brain known as the suprachiasmatic nucleus, or the SCN. The SCN can be found in the brain's hypothalamus region and receives signals from the optic nerve based on the amount of light in the environment. These signals trigger a release of hormones, which can cause sleepiness or alertness.

 

Diagram of the SCN and hypothalamus. Source: NIGMS

The process looks something like this: When the sun rises and light shines through your window and onto

your sleeping face, the optic nerve starts firing signals to the SCN. The SCN receives these signals and

then triggers the release of hormones that promote alertness and activity. Soon after, you will wake up and

begin to move about your day. Conversely, in the evening, when the sun sets and everything gets dark,

your optic nerves send signals telling your SCN to release melatonin, a hormone that promotes sleep.

Interestingly, melatonin release in teenagers has been known to be about an hour later than in adults.

This contributes to the reason that teenagers are known for being night owls and tend to perform poorer in

school when required to wake up earlier. Read more about this here. 

Graph of melatonin release throughout the night. Source: Philips

However, we now live in a world of electricity and screens, and no longer rely on the sun to dictate when

to rest. So what happens when light is no longer the primary signal for your sleep cycle? 

Recently, a group of 15 people took part in a study in which they lived in a cave with no sunlight and no way of tracking time for 40 days. They had to rely solely on their internal clocks to keep track of the passage of time, and decide collectively when it was time to sleep and wake up. While the participants spent 40 days in the cave, most people thought that it was around 30 days in total. While I would never be able to go that long in a cave without sunlight or a cool breeze, the participants considered it a relaxing time. Without light, our internal clocks do not quite follow a 24-hour cycle and can become much more varied. 

Members of the study exiting the cave after 40 days. Source: AP

On the opposite side of the spectrum, constant exposure to the light and screens that currently dictate our

sleep cycles can be very harmful and can lead to various other health problems. The SCN is very sensitive

to changes in our environments. When blue light from our phones and computers is picked up on by the

brain, it delays the release of melatonin and disrupts the sleep cycle. Short-term effects of a disrupted

sleep cycle include depression, anxiety, and problems with memory. This can lead to a greater risk of heart disease, kidney disease, and many other serious conditions in the long term. 

I know that I definitely need to be getting more sleep, especially after researching that last part. 

Our sleep cycle has evolved over thousands, even millions, of years. It is only recently that technology

has started to play a role in our lives. So the next time you start to feel tired, make sure that you put your

phone down, turn off the lights, and get some rest.