Why do we sleep?
I have to start off with an apology to my readers for the title of my post yesterday. I totally forgot to change the title before I published it – it wasn’t intended to be a trick to get you to read the post, but it sure does appear that way. Sorry! I do hope that you enjoyed the babies though…aren’t they funny?
OK, let’s get on with the actual post.
Why do we sleep?
Well the leading scientific notion is that sleep is restorative. I know, I know, you already know this – it’s nothing new. But how do we know this? What is the evidence to suggest that sleep is restorative? What goes on physiologically, biologically and neurologically to back up this claim of restoration? Could we not argue that sleep evolved along with our species out of necessity for survival (i.e. we slept to avoid getting eaten in the night)? Yes, a bit of a stretch, but it’s still something to consider.
I think before we can answer the question of why we sleep, it is important to address the physiological process of sleep – how do we start sleeping? What mechanisms come into play that force us to lie down, close our peepers, slow our breathing and tune out?
Sleep is the result of the interplay of three systems:
1) The arousal system (associated with the reticular activating system in the brain stem)
2) A slow-wave sleep centre (located in the hypothalamus)
3) A paradoxical sleep centre (located in the brain stem that has
rapid eye movement (REM) “sleep-on” neurons)
The reticular activating system housed in your brain stem assists in the regulation of incoming sensory information (i.e. it helps to shut out the outside world to the brain). We then slip into the first stage of sleep. We are easily aroused from this stage and typically toss and turn to find a comfortable position for the night. Following stage one we progress into deeper stages of slow-wave sleep (named after the increasing amplitude of the electrical signals – check out the pic). Dreams during theses stages of sleep are typically more plausible and based upon your daily events. The exception are nightmares which occur during stages 3 and 4. Progression through the four stages of slow-wave sleep takes about 35-40 minutes.
At the culmination of our slow-wave sleep cycle we move into paradoxical sleep, more commonly referred to as REM sleep – the crazy eye sleep stage. During this phase, brain activity is almost akin to that of waking (which is why it is referred to as paradoxical). Additionally, there is an abrupt inhibition of muscle tone throughout the body. Typically, we do not wake up from REM sleep, but if you are awakened during this phase you will experience a moment of paralysis which can be very disconcerting. REM lasts for about 10-15 minutes and is typically accompanied by very vivid, emotionally charged dreams.
This gives a very basic overview of the mechanisms responsible for sleep. On Monday I will address the initial question of why we sleep.
Happy sleeping (tonight).
Sherwood, Lauralee. 2006. Fundamentals of Physiology.