Feeling tired? I know I am. But what is it that makes us feel like we need to sleep? And what keeps us awake?
Within our bodies, a lot of systems work antagonistically to maintain homeostasis – that is, one action opposes another, keeping balance. There are two important forces that we use to control sleep: our sleep drive and our alerting signal. As the day goes on, the drive to sleep becomes greater and greater, so our alerting signal has to increase at the same rate to keep us awake. Sometimes this doesn’t always work in sync, which is why I often feel the need to cat-nap! When the alerting signal finally drops off, we fall asleep and our sleep drive decreases back to its original level, ready for a new day.
There are a number of hypotheses of how this works, the best characterised is based on adenosine. Adenosine is released when ATP is broken down by cells to produce useable energy. While you are awake the levels of adenosine will build up and bind to receptors on the brain cells in charge of alertness, causing them to become inhibited (less active). This means that over time you’ll have a dip in your alerting signal, so the drive to sleep will win and you will fall asleep. Overnight adenosine levels drop as your body uses less energy, and so your need to sleep is reduced also. Interestingly, caffeine can also bind to these receptors and stop adenosine from binding, explaining why you feel so awake after a cup of coffee!
Of course it’s not all as simple as it seems, there are other factors that affect your sleep, one such being daylight. Before the invention of the light bulb our body relied on natural sunlight to regulate our body clock (also known as circadian rhythm). Now we are exposed to so much artificial light on a daily basis that our brain can often find it hard to distinguish what time of day it is and whether it’s time for sleep! Cells in the front of the hypothalamus are in charge of this circadian rhythm, and they in turn receive signals from the optic nerve (which receives its information from your eyes) telling your brain how bright it is outside and whether you should be in bed or not. Interestingly, if you damage these cells, you lose the ability to regulate your body clock.
So there you have it, the science of sleep!
I’ll leave you with a quote:
“The best cure for insomnia is to get a lot of sleep”- W. C. Fields
Happy Friday everyone,
Porkka-Heiskanen T, Strecker RE, & McCarley RW (2000). Brain site-specificity of extracellular adenosine concentration changes during sleep deprivation and spontaneous sleep: an in vivo microdialysis study. Neuroscience, 99 (3), 507-17 PMID: 11029542