I laughed the first time I heard the term ‘sleep hygiene.’
I thought it had something to do with changing your sheets.
Nope. The National Sleep Foundation defines ‘sleep hygiene’ as, “a variety of different practices that are necessary to have normal, quality nighttime sleep and full daytime alertness.”
Be honest. When was the last time you had full daytime alertness without caffeine?
Do you wake up in the morning feeling like you didn’t get the sleep you wanted, even though you’ve been in bed for 7 hours?
Our understanding of why we sleep is expanding, together with the evidence that a lack of sleep can impair brain function, dysregulate appetite and food seeking behaviours, impair blood sugar management, and alter stress hormone patterns.
This article will look at three of the key roles sleep plays in our health, and how lack of sleep affects us. It will be followed on Thursday with an article on how to improve your sleep.
It’s intrinsic knowledge to any living person that sleep is critical. It is a biological phenomenon observed throughout the animal kingdom, and extraordinary evolutionary adaptations have occurred to facilitate it: dolphins, who require surface breathing, sleep by alternating one side of their brain in a sleep state, while the other side remains awake. 
In fact, looking at sleep through an evolutionary lens gives us all the information we need about its importance.
You see, in order to sleep, we need to shut off responsiveness to the external environment, to a state of partial or total suspended consciousness . For 8 out of every 24 hours, we are essentially sitting ducks: if anything speaks to the vital functions of sleep, it’s that out of all evolutionary adaptations, we risked being in an unaware and unresponsive state for one third of every day to get it.
We sleep in stages, of which there are two primary types: non-rapid eye movement (NREM) and rapid-eye movement (REM). Sleep occurs through progressive stages of depth associated with different brain waves, beginning with stages 1-4 of NREM, followed by REM. 
In NREM sleep, stage 1 is the lightest stage, stage 2 accounts for the greatest amount of total sleep time, and stages 3 and 4 are known as “slow-wave sleep”, a deep sleep stage which occurs predominantly in the first third of a sleep session. 
REM is characterised as a state in which the brain is active, but the body is paralysed . This is the stage in which we dream and consolidate memory, and it begins roughly 60-90 minutes into a sleep session, accounting for a quarter of total sleep time in a healthy sleep structure. 
The stages do not progress linearly: after slow-wave sleep, we return to stage 2 NREM before entering into REM . REM can be followed by either a return to stage 2, stage 1 or an awakening state. 
As a sleep session progresses, we spend less time in NREM stages 3 and 4, and more time in REM . Appropriate sleep duration is thus clearly important.
The question is, how much is enough?
A 2013 National Sleep Foundation poll of populations in a sample of 6 countries, showed the perceived amount required for optimal daytime functioning is around 7.5 hours. 
And we are sleeping less and less: average sleep duration in the 1960’s was over 8 hours, now we average 6.5 hours per night, with up to 20-30% getting less than 6 hours. 
Given that consecutive nights of 6 hours sleep or less is associated with cognitive performance deficits, recommendations should be based on the sleep time associated with reduced health risks and increased performance, which is 7.5-8 hours. [6, 2]
The following are three essential roles of sleep in health, and the health effects of lack of sleep:
Recently, two interesting hypotheses of why we sleep have emerged.
The first, led by University of Oregon neuroscientist Jeffrey Iliff, demonstrated that sleep allows for the clearance of waste products that accumulate in the brain during the waking, active state. 
Unlike the rest of the body, where metabolic by-products and waste are cleared by the lymphatic system and delivered to the liver for processing and excretion, the brain is not connected to, or served by, the lymphatic system. 
So how does the brain clear the waste that builds up from it’s high (20% of our daily energy requirements) metabolic cost?
What they observed was that during sleep, there is a 60% increase in the interstitial space, (i.e. the space between neurons in the brain) which allows cerebrospinal fluid (CSF) to flow into the space and remove waste products, including beta-amyloid. 
B-amyloid is the protein that degrades into the plaque formation that characterises Alzheimer’s disease.
There is an excellent Ted talk given by Jeffrey Iliff on their research, which you can see here.
The researchers termed this system of CSF waste clearance the “glymphatic system” . And the effectiveness of this glymphatic system is totally dependant on sleep, as the state in which the increase in interstitial space occurs, facilitating the removal of degraded waste products accumulated during wakefulness. 
The second hypothesis, proposed by Giulio Tononi and Chiara Cirelli, professors of psychiatry at the University of Wisconsin, is termed the ‘synaptic homeostasis hypothesis’ (SHY). The SHY posits that during sleep, the synapses between neurons are weakened, such that they conserve energy and reduce stress on nerve cells. 
During the waking state, as we encounter new environmental stimuli, synapses pass signals from one neuron to another, and the links between neurons strengthen as they seek to process, learn and remember the information. 
During sleep, however, while we lie there out cold, neurons continue to fire with as much activity and energy cost as in the waking state. 
Neurons fire spontaneously in sleep (you’ve experienced this in the randomness of dreaming), allowing the brain to decide what information is worth keeping, and what isn’t relevant or necessary . Sleep provides an opportunity for the brain to sort through what it needs to encode, and what it doesn’t. By weakening the synaptic connections, the brain can restore to a state where it can learn and adapt again, once awake. 
If the synaptic activity of an area of the brain is overexerted (like in learning), the need for sleep for that area (known as ‘local sleep’) increases. 
If we don’t sleep, or sleep adequately, then that area of the brain will take a localised “off” period . You’re awake, but you’re in a state where you’re exposing yourself to errors of judgement, mistakes and bad moods. 
These two hypotheses do not have to be looked at exclusive of each other. In fact, they could overlap. The energy cost of strengthening synapses during the day is a major source of metabolic waste in the brain, requiring the clearance of those waste products by CSF and the glymphatic system.
Both processes allow the brain to reset, clear waste, process the information from the day in order of relevance, and conserve energy to allow it to perform, learn and adapt the next day.
Put simply, you’re smarter if you sleep.
We can synopsise this in one line: lack of sleep makes you crave high carbohydrate, high calorie foods, disinhibits your food control, and makes you overeat.
It is estimated that around 44% of working adults sleep less than 7 hours per work night, a dramatic increase from 15% in 1960. 
The consequences of lack of sleep are a hormonal mess that has us headed for overeating and weight gain.
In healthy subjects, the impact on blood sugar management is comparable to a pre-diabetic state: insulin sensitivity decreases, while the function of the pancreas in secreting insulin is impaired . Other markers of insulin resistance are present, including elevated free fatty acids and increased glucose production by the liver. 
These effects are seen with anything between 4 to 5.5 hours sleep. 
The irony of ending up in this temporary pre-diabetic state, is that the very thing your body can’t handle in this state – carbohydrate – is the very thing your brain is telling you to go and eat: the desire for high carb foods increases by 30% in a sleep deprived state. 
Leptin, the hormone that signals “I’ve had enough”, decreases by 18% and ghrelin, the hormone that signals “I’m hungry”, increases by 28%, stimulating food seeking behaviours. 
This decrease in leptin levels is comparable to the decrease observed in research where healthy subjects are underfed by 900 calories per day. 
This is significant, because in the same vein that low leptin from extreme dieting causes compensatory food seeking behaviour, the decrease in leptin in a sleep deprived state results in a 22% increase in calorie intake the following day. 
The percentages here are interesting, as leptin modulates energy intake . The decrease in leptin in the sleep deprived state (18%), and the compensatory calorie intake (22%), is almost a 1:1 ratio, such is the accuracy of our inbuilt homeostatic mechanisms.
The lack of appetite regulation is not exclusively hormone-driven: neuroimaging has shown that the areas of the brain affected by lack of sleep are those which influence appetitive evaluation of caloric value, exaggerate food reward, and compromise inhibitory control over food intake and food seeking behaviour. 
Translation: you eyeball donuts with lust, eating one is never enough, and when you’re done with them, the voice in your head is screaming “more!“
Energy expenditure is also affected, further compounding the regulation of calorie intake in the sleep deprived state.
Lack of sleep actually increases energy requirements to support the additional energy cost of staying awake . The compensatory intake, however, predictably exceeds energy requirements, with a preference again for carbohydrate consumption. 
The timing of the calorie intakes is also notable: in subjects sleeping 5 hours per night, more calories were consumed after dinner than at any other single meal. 
While lack of sleep increases energy requirements, it can lead to decreases in physical activity. 
More alarmingly, comparing subjects sleeping 5.5 and 8.5 hours undergoing a weight-loss diet, subjects lost the same amount of weight, but the 5.5 hour sleep group lost 55% more lean body mass! 
So we have dysregulation of appetite control, with an increase in desire for high carb foods, which occurs in the midst of a blood sugar train wreck. Add to that a decrease in lean mass, which lowers metabolic rate, and a decrease in energy expenditure, and what do you get?
A perfect recipe for weight gain, adding sleep to the list of environmental factors with a role in the development of obesity and diabetes.
Circadian rhythm is more colloquially known as your ‘body clock’; internal mechanisms which regulate our sleep-wake cycles, and the physiological fluctuations that occur around that 24-hour cycle.
This means that our circadian rhythm, built around the 24-hour sleep/wake and dark/light cycle, regulates everything from sleep patterns, to appetite, carbohydrate and fat metabolism, to wakefulness and alertness. 
In healthy people, a normal circadian rhythm can be defined by fluctuations in cortisol, a stress hormone secreted by the adrenal glands, and melatonin, the hormone secreted by the pineal gland that regulates sleep.
For cortisol, normal fluctuations means that cortisol rises sharply by up to 75% in the morning, peaking about 30 mins after waking (known as the ‘cortisol awakening response’, or CAR). 
The CAR, in a regular circadian rhythm, increases alertness and wakefulness, providing a boost of energy so you can get up and attack your day. 
Cortisol and melatonin have an inverse relationship in a regular circadian rhythm. Cortisol rises during the latter stages of the night, peaks in the morning, takes a steep decline over the 3 hours after waking, and gradually declines over the course of the day to it’s lowest point at the onset of sleep. 
Melatonin, on the other hand, is released during the biological night, providing the body with an internal signal that it is nighttime, and increasing our propensity to sleep. 
It is sensitive to seasonal changes in light, so it not only responds to the onset of darkness, but also to the duration of the night, regulating sleep accordingly. 
The melatonin signal regulates the circadian organisation of other important physiological functions, including the immune systyem, antioxidant defences, haemostasis and glucose regulation. 
Due to the presence of melatonin receptors with such important functions throughout the body, melatonin disruption is now being investigated for it’s role in the development of metabolic syndrome, type-2 diabetes, and cancer. 
So if you knock your circadian rhythm out of whack, you end up with dysregulated melatonin and cortisol patterns, and downstream negative effects on energy, tiredness, appetite and overall health.
The question begs, what would knock this biological rhythm out of balance?
Light. And stress.
In 99% of people, exposure to light in the hours before bed suppresses melatonin onset, and shortens melatonin duration by 90%. 
In effect, decreasing your body’s internal perception of how long you’ve been asleep.
Excessive evening light exposure also messes with sleep quality, by increasing the time it takes to enter rapid eye movement (REM) sleep, and increasing non-REM brain wave activity. 
The effect is global: both sleep structure, and sleep quality, are impacted dramatically. Melatonin onset is suppressed, secretion is suppressed during the night, and duration is reduced by 90 mins. 
Cortisol is also affected by light, or rather, lack thereof: lack of exposure to natural light during the daytime raises cortisol levels, which is associated with depressive symptoms. 
And while the focus of this article is sleep, lifestyle goes hand-in-hand with everything, so it would be remiss not to mention the impact that the daily stressors of modern life have on cortisol.
Work, money worries, relationships, the pace of life, you name it: we have more daily stressors now than ever before, with estimates that life’s pressures have increased 30% since 1983. 
This is important, as the cortisol disruption caused by stress in turn increases the desire for, and intake of, hyper-palatable, high calorie foods. 
Basically, when you’re stressed, you eat shit.
Feeling groggy in the morning despite 7-8 hours of bedtime might be the best indicator you have that your internal body clock is off. The knock on effect of an offset circadian rhythm should be looked at in conjunction with the appetite dysregulation and blood sugar mismanagement that typifies a lack of sleep.
Add in some work deadlines, and there’s the perfect stress hormone mess to have you sagging on the couch at 7pm and reaching for the Domino’s menu.
I think that on an intrinsic level, the importance of sleep is something known to us, buried in our hardwiring.
We don’t need science to tell us that sleep matters.
But with our modern lifestyles, it’s the one thing that gets shoved down the priority list.
So we do need science to hammer the point home: tuck in and get your kip.
In the follow-up to this article, I’ll show you steps you can take to improve your sleep.
As always, let me know your thoughts, comments, or questions.
Header image courtesy of: http://1.bp.blogspot.com/sleep+and+dreams+and+creativity.jpg