Why do we sleep? Although it seems a pretty simple question, experts who study sleep are not certain about its exact function. And there are many theories. Some believe that it is a time of cellular restoration . From childhood I subscribed to this theory, complete with a creepy vision of little elves traveling all over my body fixing things, whilst my consciousness slumbered on. Yeh, I know. The downside to this theory is that when we are in the rapid eye movement (REM) phase of our sleep cycle our bodies are highly active, leaving little time for restoration. And yet paradoxically we need REM sleep to feel restored. So this idea could use some work. Others have suggested that sleep provides a time when we can save up on energy . After all, as animals of the daytime, humans do their things in the light, so why not benefit during the times when there’s nothing to do? But predators lurk in the dark, and so for this theory to work, the energy savings should be pretty high or this strategy would have evolved away ages ago. Turns out the energy savings calculation has been done, and the energy saved is roughly equivalent to the amount of energy in one hot dog bun . What a bargain! Hardly worth the expense of becoming a favourite midnight snack. The third, and rather attractive, theory is that sleep is necessary for learning and memory . There has been a lot of Neuroscience research lately that has examined how we learn, and many studies have indicated that a nights’ sleep is crucial to consolidating our learning into memories that we hold on to [4, 5]. For example, give two groups of people some things to learn and then give one group a snooze and the others not. And viola’ – the ones that slept out-performed those that did not . Sounds good. The logical extension of this, however, is that we have to sleep a lot because we have such awesome, complex, and powerful brains. But that lazy cat sleeps about 12 hours a day, and the notoriously [not so] quick-witted North American opossum about 18 hours . Hmmm. So, while attractive, this theory seems to need considerable refinement. Thus, the jury is still out on why we sleep.
What we probably know best about sleep is that we all need it, and most of us have, or have heard of, at least one amusing story about the lengths that we will go to in order to ensure a good night’s rest. Despite this we mostly have a rather unrealistic attitude about the need for sleep, and it is common for sleep deprivation to be lauded as a personal strength. Haven’t we all heard, ‘successful people only sleep 4 hours a night’ or ‘you can sleep when you are dead.’ New parents will know all too well the perils of sleepiness, and sleep-deprived people have been known to do some pretty funny things. But sleepiness has a dark side, and even small changes in the amount we sleep can have dire consequences. For example, the rate of car crashes increases by 20% on the Monday following the one hour loss of sleep when we change the clocks forward in spring . And if that wasn’t scary enough to make you want to stop all this seasonal clock changing, here’s another one: the risk of heart attack goes up by 5% in the 3 weeks after we lose an hour’s sleep in the switch to daylight savings . This tells us is that sleep is important for our brains to attend properly to daily activities. Indeed, most of us already know this anecdotally, and a lot of study has been done on sleep deprivation and our performance. What I’d like to cover briefly here instead are the health implications of our basic daily, or circadian (‘about a day’) rhythms, operating under normal circumstances.
Did you know that the morning is the deadliest time of the day? During the hours between 6 AM and 12 noon you are three times more likely to have a heart attack or stroke than in the hours after midday [6, 7]. So, why this risk in only one part of the day? Because we are most active during the light hours, we might imagine that’s when things are the most likely to go wrong. Fair enough. But if you think about the things that you do in a day, have you ever noticed that you are better at certain things at certain times of the day? In general, we are most alert in the mid morning, whereas our coordination and reaction time peak in the early afternoon, and our strength and cardiovascular efficiency peaks in the late afternoon . If you, like me, enjoy a bit of competitive sport, you may already be rethinking your workout or competition schedule.
This timing works with all of our body systems – there are certain things that our bodies do, behind the scenes and completely out of our control, on a reasonably strict circadian schedule. Besides the things mentioned above, our natural subconscious circadian rhythms are also why we poo in the morning, and get sleepy at night. The master regulator, of course, is our brain, because it is able to know the time of day and consequently to control our hormones. The brain gets its information from special cells in our retinas that sense light and pass that information on to the brain. These retinal cells are completely different from the rods and cones that we use for vision, in fact they transmit no visual information at all, only the signals of whether it is light or dark [9, 10]. Thus, people who become blind because of photoreceptor degeneration, such as in retinitis pigmentosa, are still able to sense daylight and have normal circadian rhythms [10, 11]. Pretty cool, eh?
Now, on to the hormones that the brain enslaves. Hormones are powerful chemicals in our blood that regulate vital functions of our bodies, and as I’ve already said our brains control the hundreds, if not thousands, of hormones circulating around our bodies all the time. The amount of hormones changes throughout the day (as dictated by our brain), and each type of hormone has a particular time (or times) in the day when it is highest and lowest, and in between of course it’s either on the rise or decline. During the peak times, our bodies are doing the things that the hormones instructs, such as being alert or strong or sleepy.
OK. So you may be asking: if the morning is the riskiest part of the day for heart attack and stroke, what hormones are around then? And what are our bodies doing that might explain the dark side of this otherwise bright part of the day? For one, our blood pressure rises sharply right around 7AM, which puts extra load on our blood vessels, and might be one explanation . Incidentally, this is why blood pressure medicine is most effective when taken in the morning. Indeed, a lot of research has gone into developing medical interventions to try to prevent this dangerous rise. But heart attack and stroke happen because the blood vessels in the heart or brain can become blocked by tiny blood clots. So, in a new study, researchers asked whether hormones that cause blood clotting might be higher in the morning. In this study, reported last week, Neuroscientists found that a chemical, plasminogen activator inhibitor, which enhances blood clotting, is highest in the morning, which in turn means that the likelihood of blood clots is also highest at this time .
So, why not just sleep-in and bypass this scary part of the day? Alas, these changes are controlled by our internal body clock, our circadian rhythm, which operates through an elegant mechanism that senses the light and dark times of day, not whether we are awake or asleep; which means that they happen whether you are in bed or not. So get up, shake out the elves, and enjoy the day – what have you got to lose?
Much credit for the content goes to Professor Russell Foster from Oxford University , who I was lucky enough to meet at a conference last month. Not just a phenomenal scientist – a fantastic person.