Light – a doubled-edged sword for sleep

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There have been reports in the UK press this week of a report that informs us that we are sleeping quite a lot less than we did a few decades ago, and the problems that may befall us as a result. Here’s a typical story which appeared in the UK broadsheet The Daily Telegraph.

Short sleep and sleep deprivation have been linked with a variety of mental deficits (including problems with decision-making and memory) and physiological changes too including an increase in levels of inflammation in the body [1]. Inflammation in the body may participate in the poor functioning of hormones such as insulin (insulin resistance) and leptin (leptin resistance) – both of which are implicated in obesity. In one study, just one night of about 4 hours sleep was enough to impair insulin functioning [2]. Short sleep also has been found to lower levels of the hormone leptin [3] and increased levels of the stress hormone cortisol [4] (excess cortisol can cause fat deposition, usually around the midriff).

Sleep deprivation may impair the rate at which we burn fat. In one study, disrupting men’s sleep with an alarm clock and allowing them only an average 6½ hours sleep, compared to no alarm clock and 8 hours sleep, caused their metabolism of fat of to fall by two-thirds [5]. Perhaps not surprisingly, lack of sleep can also bring about unhealthy changes in body composition. In one study, shortened sleep was found to reduce fat loss but increase muscle loss [6]. This effect may have something to do with the ability of sleep deprivation to increase levels of cortisol, as this hormone predisposes to muscle loss as well as fat gain.

Bearing all this in mind, it’s perhaps no great surprise that some researchers have suggested changing sleep patterns over recent decades might be an important but under-recognised factor in the burgeoning rates of obesity and chronic disease [7].

I don’t believe that all people who tend to get short amount of sleep are putting themselves at risk: I suspect there exists people who generally thrive on short sleep. However, I also believe that, as a whole, many individuals are ‘pushing it a bit’, and simply are not getting the amount or quality of sleep required to be at their best or enjoy the best of health.

I sometimes ask people how many hours of sleep they feel they need to feel and function optimally. Then I ask them how many of hours of sleep they get in the week. Almost invariably, the latter figure they give me is one hour less than the former.

One thing that has been highlighted (excuse the pun) in the recent reports is sleep is how our natural body clock may become disrupted by an ever-lengthening day, as well as our exposure to light. It reminded me of a piece of research in which a group of adults were monitored in their normal environments (with electrical lighting, TV and electronic devices), and also had their levels of the ‘sleep hormone’ melatonin assessed [8].

It turned out that in their normal environment, melatonin secretion was generally delayed at night. Also, at 8 o’clock in the morning melatonin levels were still raised, and remained high for several hours after they got up. High levels of melatonin are good when we want to sleep, but not so good if we want to feel fully energised and ‘raring to go’.

This ‘delaying’ effect on melatonin is not too much of a surprise, as previous research has produced similar findings. In one study, exposure to room lighting prior to bedtime delayed melatonin secretion by 90 minutes on average, and reduced the overall amount of melatonin too [9]. In another study, light from laptops and tablet devices was found to suppress melatonin too [10].

The study subjects were then taking camping for a week, with the only light they were exposed to coming from the sun and a campfire. By the end of the week, the study subjects had their sleep ‘synchronise’ with the setting and rising of the sun. The sleep-wake cycle shifted by about 2 hours back (earlier). And in the morning, melatonin levels were found to be low (as they should be).

Getting more natural light during the day is very likely to have been a significant factor here, I think. Melatonin is actually made from the brain chemical serotonin, the production of which is stimulated by sunlight.

It’s generally impractical for many of us to live ‘under canvas’, but there are steps we can take to give our brain the best chance of being ‘in sync’ with natural sleep cycles. Two very simple things that might help is to be mindful of getting exposure to natural sunlight each day, and to think about importance of keeping light exposure to a minimum in the evening.

It’s the blue part of the visual spectrum that appears to be most beneficial during the day, but also is the most disruptive at night. One solution might be to don a pair of blue-light-blocking (orange) sunglasses.

Orange-tinted lenses filter out blue light and, theoretically, should help sleep. In one study, the use of orange-tinted glasses 3 hours prior to sleep improved both the quality of sleep and subsequent mood [11].


1. Meier-Ewert HK, et al. Effect of sleep loss on C-reactive protein, an inflammatory marker of cardiovascular risk. J Am Coll Cardiol 2004;43(4):678-83

2. Donga E, et al. A single night of partial sleep deprivation induces insulin resistance in multiple metabolic pathways in healthy subjects. J Clin Endocrinol Metab. 2010;95(6):2963-8

3. Spiegel K, et al. Impact of sleep debt on physiological rhythms. Rev Neurol (Paris. 2003;159(11 Suppl):6S11-20

4. Reynolds AC, et al. Impact of five nights of sleep restriction on glucose metabolism, leptin and testosterone in young adult men. PLoS One 2012;7(7):e41218

5. Hursel R, et al. Effects of sleep fragmentation in healthy men on energy expenditure, substrate oxidation, physical activity, and exhaustion measured over 48 h in a respiratory chamber Am J Clin Nutr 2011;94(3):804-8

6. Nedeltcheva AV, et al. Insufficient Sleep Undermines Dietary Efforts to Reduce Adiposity. Ann Intern Med. 2010;153(7):435-41

7. Van Cauter E, et al. Metabolic consequences of sleep and sleep loss. Sleep Med 2008;9 Suppl 1:S23-8

8. Wright KP, et al. Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle Current Biology, 01 August 2013

9. Gooley JJ, et al. Exposure to room light prior to bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab 2011;96(3):E463-72

10. Wood B, et al. Light levels and duration determines the impact of self-luminous tablets on melatonin suppression. Applied Ergonomics 2013;44(2):237-240

11. Burkhart K, et al. Amber lenses to block blue light and improve sleep: a randomized trial. Chronobiol Int 2009;26(8):1602-12






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