Lazy bones: The importance of sleep

By Sarah Lancaster
Consultant Orthopaedic Trauma Surgeon and Deputy Clinical Director, Oxford University Hospitals

We need to sleep. Sleep is a time when our bodies repair and regenerate, our metaphorical batteries recharge. The benefits of sleep have gained increased interest recently, but many of us still don’t prioritise this vital necessity.  

Why do we sleep?

All animals with a brain have to sleep. Although it’s still not known exactly why sleep is necessary, it is obvious this process has to happen for survival. This is illustrated by a rare disorder, known as fatal familial insomnia, which exhibits autosomal dominant inheritance. Sufferers of this condition demonstrate a progressive inability to sleep, which leads to cognitive decline, ataxia and always death.

What happens when we are asleep?

If you have used a sleep tracker, you are probably familiar with the stages of sleep. Broadly speaking, there are two types of sleep; rapid eye movement (REM) and non rapid eye movement sleep (NREM). NREM is further divided into N1, N2 and N3 cycles. Sleep generally starts with a period of wakefulness, followed by N1, which is light sleep. Breathing regulates and there is a progression to the N2 stage, which is deeper sleep. During the N2 stage, electroencephalogram (EEG) readings demonstrate sleep spindles, which are bursts of electrical brain activity, thought to be involved in memory consolidation. The heart and breathing rates slow. The N2 stage is the longest of the NREM stages, and is followed by N3, which is the deepest level of sleep. It is during N3 sleep that ‘slow waves’ dominate the picture, and it is thought to be the time of best physiological repair. Slow waves sweep over the brain and are believed to have a role in clearing the brain of the protein beta-amyloid, the accumulation of which is associated with dementia development¹.

REM sleep then follows; this is a period of relative brain activity, when dreaming occurs, and the heart and breath rates increase. REM sleep increases over the course of the night, with most REM sleep experienced prior to waking.

If you’ve ever wondered why a problem seems a little less challenging after a night’s sleep, for example a tricky pilon to tackle the next day, it is likely because your brain has had time to digest the information, allow it to consolidate and made sense of it during your sleep.

What makes us sleepy in the first place?

Adenosine, which is a by-product of metabolism, builds up in the brain throughout the day and binds to adenosine receptors. As this accumulation increases, the so called ‘sleep pressure’ builds, making us feel more and more sleepy. This process is underpinned by the Circadian rhythm, which in humans is a roughly 24-hour internal clock, keeping the overall sleep/wake cycle regular. Caffeine is an adenosine receptor antagonist, thereby reducing drowsiness. With a half-life of up to seven hours, it is not recommended that caffeine is consumed late in the afternoon, to avoid stimulating the brain prior to sleep. 

There are many things that influence the Circadian rhythm, one of the most obvious being light and dark. Aside from the build up of adenosine, melatonin is released from the pineal gland during darkness, usually late in the evening. Melatonin binds to two types of receptors in the brain, promoting drowsiness. Many of these receptors are found in the suprachiasmatic nucleus, which is a primary region in the brain for regulating the sleep/wake cycle.

Early Birds and Night Owls

Some people naturally prefer to get up earlier, and therefore head to bed earlier in the evening. These individuals are called ‘Early Birds’. Conversely, others prefer to wake later, remaining much more alert later into the evening, and feel sleepier later in the day; the ‘Night Owls’. These two distinctions are called chronotypes. Which chronotype you prefer largely depends on genetic factors, but it can be influenced by lifestyle and aging, with older adults tending to shift their awake and alert phases to much earlier in the day, in comparison to teenagers who tend towards Night Owl behaviour. It is thought that this natural variation had a previous evolutionary advantage, as it meant that someone within a tribe was always awake, to keep watch for predators, keep the fire burning and so on. It is akin to a naturally occurring shift pattern for survival. The standard 9am - 5pm working day we have adopted in modern society likely favours the Early Birds, and I suspect most surgeons are natural Early Birds.

Is there evidence that insomnia is detrimental?

Insomnia has been shown to impact quality of life in those affected², with previous emphasis being on mood and psychological impact. However, psychological quality of life is not the only consequence; a large population study in Norway, published in 2014, demonstrated that yes, psychological disorders were highly associated with insomnia, but osteoporosis, headaches, asthma and myocardial infarction were also associated. There was also a relationship with lack of sleep and angina, hypertension, obesity and stroke³. Further evidence points to insomnia as a cause of type 2 diabetes⁴.

Additionally, an obvious but often overlooked consequence of insomnia is the fact that those not getting enough sleep at night are more likely to have accidents during the day, perhaps up to four times the number of those who have no trouble sleeping².

Specifically, how does sleep impact bone health?

In 2012, a US group investigated bone formation in sleep deprived rats. The bone of sleep deprived rats demonstrated reduced bone osteoid, both volume and thickness, compared with normal controls. Osteoblast numbers were lower in the sleep deprived group, but osteoclast numbers remained the same across the two groups, with bone resorption (i.e. osteoclast activity) also being unchanged across both groups. Reduced bone formation paired with the same activity in osteoclasts therefore resulted in osteoporosis in the sleep deprived rats⁵.

Aside from rats, human population studies have demonstrated a link between lack of sleep and reduced bone health. A public health study from China in 2011 observed the sleeping habits of 602 women, categorising them into sleep durations of less than 5 hours, 6, 7, 8 hours, or 9 hours or more. Their bone mineral density was assessed using dual-energy X-ray absorptiometry (DEXA) and the investigators found statistically significant reductions in bone mineral density, for those aged 45 years and older, in sleepers of 6 hours or less in comparison to the longer sleepers⁶.

These findings substantiated those previously published in 2007 by Speckler et al. DEXA scanning and other volumetric parameters were observed in 1,146 individuals, male and female. Sleep deprivation in this study was defined as less than 6.5 hours per night, and individuals self-categorised into sleep deprived or sleep adequate. Bone mineral density was found to be reduced in sleep deprived females, and other bone volumetric parameters (for example cortical area of the wrist) tended to be lower in sleep deprived males, although not all these findings reached significance threshold⁷.

Summary

Sleep should be seen as a wonder drug, which, like exercise, has benefits to both short and long term health. Poor sleep is associated with increased risk of dementia, diabetes, obesity, osteoporosis. Alcohol, although initially likely to cause drowsiness, ultimately reduces the brain’s ability to transition through the various stages of sleep, particularly affecting REM sleep, so reducing alcohol intake is likely to improve sleep quality. It is easier said than done, but prioritising sleep and practising good sleep hygiene is probably more important than we think for our mental and physical health. Patients recovering from surgery should be encouraged to get plenty of sleep as their tissues repair. Stress, shift patterns and constantly changing working environments will disrupt our sleep patterns, so, as part of our own wellbeing, we should try to get enough sleep so that we can make the most of being awake.

References

1. Wunderlin M, Zust MA, Feher KD, Kloppel S, Nissen C. The role of slow wave sleep in the development of dementia and its potential for preventative interventions. Psychiatry Res Neuroimaging. 2020;306:111178. 
2. Roth T. Insomnia: definition, prevalence, etiology, and consequences. J Clin Sleep Med. 2007;3(5 Suppl):S7-10.
3. Sivertsen B, Lallukka T, Salo P, Pallesen S, Hysing M, Krokstad S, Simon Øverland. Insomnia as a risk factor for ill health: results from the large population-based prospective HUNT Study in Norway. J Sleep Res. 2014;23(2):124-32.
4. Liu J, Richmond R, Bowden J, Barry C, Dashti H, et al. Assessing the Causal Role of Sleep Traits on Glycated Hemoglobin: A Mendelian Randomization Study. Diabetes Care. 2022;45(4):772-81.
5. Everson CA, Folley AE, Toth JM. Chronically inadequate sleep results in abnormal bone formation and abnormal bone marrow in rats. Exp Biol Med (Maywood). 2012;237(9):1101-9.
6. Fu X, Zhao X, Lu H, Jiang F, Ma X, Zhu S. Association between sleep duration and bone mineral density in Chinese women. Bone. 2011;49(5):1062-6. 
7. Specker BL, Binkley T, Vukovich M, Beare T. Volumetric bone mineral density and bone size in sleep-deprived individuals. Osteoporos Int. 2007;18(1):93-9.