Sleep is complex!
Sleep is a complex phenomenon and is characterised by distinct behavioural, physiological and neurophysiological aspects. As humans we spend about a third of our lives sleeping, and on the whole many of us take sleeping for granted. However, if you have ever been sleep deprived you will know how utterly draining it can feel. Poor or lack of sleep impacts all aspects of our wellbeing therefore it is important to ensure you get enough good quality sleep so that you wake up feeling refreshed and ready to face the day. The recognised sleep disorder, insomnia becomes a major preoccupation for those people suffering from it, and poor sleep is responsible for many accidents, and is associated with ill health and psychological problems. Evidence suggests that sleep deprivation is a risk factor for developing health conditions such as type II diabetes, obesity and hypertension as well as having a negative effect on our mental health.
Sleep for many people is a regular behaviour that occurs at certain times of the day, normally when it is dark, and lasts for a number of hours. Some people arguably require more or less sleep than others, but on the whole humans tend to require around 8 hours sleep per day. Our sleep requirements tend to follow a circadian rhythm; ‘circadian’ comes from the Latin meaning ‘about a day’, and rhythm is said to have a period of 24 hours. As well as our body temperature during sleep following a circadian rhythm, secretion of the ‘sleep’ hormone melatonin also follows the same rhythm such that it is at a much higher level in the blood during the hours of darkness when compared to the hours of daylight. In fact, daylight triggers the switching off of melatonin production and hence we wake up. This is why it is so important to get access to daylight in the morning to avoid feeling sleepy during the day. A good way to enhance the circadian rhythm process and to feel alert is to go for a walk outside in the morning after sunrise and soon after waking.
It is normal for the body to secrete more melatonin during the winter months due to there being more hours of darkness than the summer months. Some people suffering from seasonal affective disorder (SAD) are more acutely affected by the lack of daylight in the winter, which can lead to feelings of depression. Such people sometimes can alleviate their symptoms by sitting in front of extremely bright light to simulate daylight. Whilst we are awake there is a steady increase in the body of chemical compound called adenosine, such that as its level increases it brings on drowsiness and prepares us for sleep. During sleep it is broken down, ready to start again the following day. Caffeine and nicotine block the effect of the compound and therefore can delay the onset of sleep.
Phases and patterns of sleep
We all tend to go through two main phases of sleep with a transition period in between. Rapid-eye movement (REM) is our first phase of sleep where our eyes are moving and we are usually dreaming. (this type of sleep is also known as active sleep), non-rapid eye movement (NREM), before moving into deep sleep. Bouts of deep sleep occur early in the night and bouts of REM sleep get longer as the night progresses.
During the course of our lives, our pattern of sleep changes in a variety of ways. Infants sleep an average of 16 hours per day, teenagers nine hours and adults between five and ten hours. The sleep of older people tends to be more interrupted during the night by periods of wakefulness. Napping also plays a role in sleep quality, where older people may nap in the evening and therefore interrupt their main sleep, whilst adults may nap in the afternoon. Evidence suggests that the later you nap the more interrupted your night time sleep is. So I you want a good night’s sleep it is best not to nap close to bedtime.
The function and importance of sleep
Sleep provides restoration and recuperation. But what does this really mean?
1. Sleep has a role in maintaining homeostatic equilibrium. A sleep deficit of 3-4 hours per night affects the body’s ability to process carbohydrates, manage stress and maintain a proper balance of hormones
2. Sleep preserves energy
3. Sleep facilitates tissue repair
4. It has been suggested that REM sleep consolidates memory and improves learning
5. There is evidence to suggest that sleep plays a role in our body’s defences against infection.
How does diet effect sleep quality?
As already mentioned caffeine inhibits the effects of adenosine and therefore delays the onset of sleep, but how do other aspects of our diet effect our sleep and what if anything, can we consume to improve our quality of sleep?
A review paper (St-Onge, Mikic and Pietrilungo, 2016) examining the role of food consumption on sleep found that intakes of macronutrients (carbohydrates, protein and fat) potentially had an effect on sleep quality, whist some specific foodstuffs i.e. milk, tart cherry juice and kiwi fruit which had been previously recognised as having an effect on sleep quality, did in some cases have a positive effect.
Studies that looked at the effects of a high or low carbohydrate diet on sleep quality, found that both types of diets were associated with changes in sleep patterns. Generally, a diet high in carbohydrates led to less good quality deep sleep, but REM sleep significantly increased. If the carbohydrates were of a high glycaemic index (GI) and consumed 4 hours before bedtime then participants also found that they could fall asleep quicker, than if consumed 1 hour before bedtime. Although it is not clear, it is believed that high carbohydrate intake is related to increased serotonin synthesis, a precursor for melatonin, and this is optimised between 2-4 hours after consumption of a high carbohydrate meal. Conversely a low carbohydrate meal (higher in protein) showed a reduction in REM sleep but deep sleep was increased. High fibre dietary intake was associated with more deep sleep and less time spent in first stage REM sleep. Additionally, a diet high in sugar and fat and low in fibre was associated with more bouts of wakefulness during the night.
There have been some studies that have investigated the benefits of certain foodstuffs on sleep quality. The consumption of a malted drink has received much attention, and there is evidence to suggest that in older adults, total sleep time and greater sleep continuity improved after the consumption of a malted drink before bedtime. The effects were not seen in younger adults. It could be argued that as melatonin production declines with age the effects of the malted drink were greater in the older adults than the younger, but more research is needed. It could be argued that it is the vitamins and minerals present in malted drinks that provide the effects on sleep quality. There is emerging clinical evidence to suggest there is an association between vitamin and mineral deficiency and disrupted sleep. People with low serum vitamin D levels found that by taking a supplement the time it took them to fall asleep improved and sleep duration increased. However, the mechanisms for this are not yet clear, and anyone taking vitamin D supplements in the UK should follow the recommended guidelines of 10 µg/day, or consult a GP if they would like further advice.
There is also evidence to suggest that B vitamins have a role to play in sleep quality. Vitamin B12 affects plasma melatonin concentrations and contributes to the light-dark cycle process, as well as being associated with improvements in sleep quality and alertness. Vitamin B6 serves as a cofactor in the synthesis of serotonin, but there is no evidence to support an effect of supplementation on sleep quality.
The consumption of natural melatonin-enriched milk, obtained by milking cows at night can have an effect again on older adults. One study found that melatonin-enriched milk improved sleep efficiency and reduced the number of awakenings in older adults diagnosed with insomnia, whereas another found no effect on sleep pattern when compared to normal milk.
Kiwifruit has been found to be beneficial in increasing total sleep time and quality of sleep in adults if two are eaten one hour before bedtime. It is thought that the high anti-oxidant, serotonin and folate content of kiwifruit contributes to its effectiveness as a sleep aid, however more research is needed to support this hypothesis. Another fruit with a high serotonin content is tart cherries, some studies the authors reviewed identified that different varieties of tart cherries had differing effects on sleep quality, largely correlated with the concentration levels of melatonin in each variety. Those with higher concentrations improved sleep quality more than those with lower concentrations, however more research is needed in this area.
Sleep is effected by our diet and as a nutrition professional, I would recommend a diet rich in a variety of fruit and vegetables, consumption of wholegrains which are high in fibre and swapping saturated fats for unsaturated healthy fats. Protein intake should contribute to around 15% of your daily calorie needs and you should try to increase your proportion of plant based proteins. It’s important to eat at regular intervals and ensure you don’t consume a heavy meal close to bedtime, but the consumption of a milky malt drink may help you to drop of quicker.
St-Onge, Marie-Pierre, Mikic, Anja, and Pietrolungo, Cara, E., (2016). Effects of Diet on Sleep Quality. American Society for Nutrition. 7, 938-49. Doi:10.3945/an.116.012336.