Thursday February 12 2015
Lack of sleep can impair the immune system
"Indulging in a power nap can repair the damage caused by a lack of sleep," the Daily Mail reports. But the study that prompted the headline is very small – involving just 11 healthy young men.
It has long been known that a lack of sleep at night can have a negative impact on the immune system and stress levels.
Researchers wanted to see if two short naps during the day, each lasting 30 minutes, could repair some of the damage caused by a poor night's sleep of just two hours.
They measured biological indicators (biomarkers) such as stress hormones, and then compared them to controls in an attempt to gauge the effects of the short nap.
One of the three stress hormones measured was increased the day after the men were sleep-deprived, but not if they were allowed to take naps. The level of a protein involved in immune responses (Interleukin-6, or IL-6) was reduced after little sleep, but not if the men had naps.
The implications of these findings are unclear. Measuring one immune biomarker, such as IL-6, does not provide any insight into whether the immune system has "recovered", as it is involved in both activating and dampening the immune system.
Nor does this study show that naps relieve stress. The level of one stress-related hormone, noradrenaline, was increased after sleep deprivation, but this may have been affected by other factors.
So the results of this small study do not show whether naps improve the immune system or the body's response to stress.
If you are struggling with daytime sleepiness, you may need to improve the quality and duration of your sleep during the night.
Where did the story come from?
The study was carried out by researchers from the University of Paris Descartes and the Institut de recherche biomédicale des armées.
It was funded by the insurance company RÉUNICA and the Societé Française de Recherche et Médecine du Sommeil.
The study was published in the peer-reviewed Journal of Clinical Endocrinology and Metabolism on an open access basis, so it is free to read online.
The Daily Express informed readers that, "Even if you only get two hours of proper sleep, a half-hour snooze will relieve stress and bolster the immune system by restoring hormones and proteins."
But we can't say conclusively that a nap can do either of these things based on the results of this small, short-term study.
Only one of the three stress-related hormones tested was raised if the men did not take a nap. There are other reasons this could occur, and it was not clear if the study ruled these out.
The Express also failed to point out that this study was conducted on just 11 healthy young men over three days.
The Mail Online reported the study more accurately, but did not point out any of the limitations of this kind of research.
What kind of research was this?
This was a randomised cross-over study that aimed to look at whether naps could counteract the effect of restricted sleep on specific markers of stress and immune system response.
A group of healthy male volunteers were studied after their sleep was restricted to two hours. In one session they were allowed naps afterwards, but naps were not allowed in the other session.
The researchers measured various stress hormone levels and one immune system protein called IL-6, looking at whether the restricted sleep and naps had any effect on these levels.
The study design allows comparison of the same group of people under different conditions. This type of study needs to be careful to make sure the effects of one set of conditions do not carry over on to the other period, which is why the researchers need to allow a period of "washout" time between the two sessions.
A study of this nature does not have a separate control group – participants are compared to themselves under different conditions; in a sense they serve as their own controls. This can make it easier to detect differences that result from the conditions, as the groups being compared are essentially the same.
What did the research involve?
Eleven young men were recruited to the study through advertisements in the hospital and university campus. They were aged between 25 and 32, had a body mass index (BMI) within the healthy range of 19 to 25, and were non-smokers.
All were considered to be healthy and none had depression, anxiety or emotional distress according to a commonly used measurement tool (the Hospital Anxiety and Depression Scale). The men normally slept seven to nine hours a night on average and did not report any sleeping problems.
The men had two admissions to the sleep laboratory in random order. In the "sleep-restricted" admission, the volunteers slept from midnight to 8am on the first night, were restricted to sleeping from 2am to 4am on the second night, and were then allowed to sleep from 8pm until they woke up on the final night.
They were not allowed to sleep at any other time and were kept awake by staff with films and games.
The same night-time sleep protocol was used for the "sleep restriction plus nap" admission, but the volunteers were allowed to have a 30-minute nap at 9.30am after the restricted night's sleep, and again at 3.30pm.
The men were asked to try to be asleep from midnight to 8am for a week before the admissions, and to record their sleep in a daily diary.
During each three-day stay, their activity levels were monitored and they were provided with meals up to a maximum of 2,500 calories a day. They were not allowed to have any:
A monitor that records brain electrical activity (an EEG) was attached to each participant for the duration of each admission to record whether they were awake or asleep.
Urine samples were taken every three hours between 10am and 7pm to test for three hormones that help regulate the body's response to stress: noradrenaline, adrenaline and dopamine.
Salivary samples were taken every two hours while the men were awake, and tested for Interleukin-6 (IL-6) levels. IL-6 is a protein that is part of the immune system. It plays a complex role – it stimulates the body's immune system to react, but also reduces inflammation, depending on the circumstances.
What were the basic results?
After the sleep-restricted night, the level of noradrenaline in the men's urine was 2.5 times higher in the afternoon than at the same time of day after a night of eight hours' sleep. There was no increase in noradrenaline if they had been allowed the naps.
There was no significant difference between the sleep-restricted and non-restricted days, or with and without naps, in terms of the levels of adrenaline, dopamine or testosterone in the men's urine samples.
Levels of IL-6 were significantly lower at 10am and 7pm after the restricted sleep night compared with after eight hours' sleep. The levels were not lower if the men had napped.
After the recovery night's sleep, adrenaline and dopamine levels were increased in the afternoon in the "sleep-restricted" session, but not in the "sleep-restricted plus nap" session. IL-6 salivary levels were the same as after the eight hours' sleep in both sessions.
How did the researchers interpret the results?
The researchers concluded that: "Napping as a countermeasure to sleep restriction could, in addition to benefits on alertness, improve neuroendocrine stress and immune recovery with a potential prophylactic long-term effect on cardiovascular health."
They acknowledge that interpretation of the varying levels of the immune system marker IL-6 is complex as it can be a sign of inflammation, but it may also be involved in preventing inflammation.
This was a small study that is interesting on an intellectual level, but has little real-world practical applications or implications.
This study found the levels of one stress-related hormone (noradrenaline) increased after restricted sleep, but not if the men had naps. However, this does not prove that naps "relieve stress", as the media has indicated.
Noradrenaline is just one of several hormones that fluctuate during the day in response to a variety of bodily functions. Although it is known as one of the stress hormones, this refers to stress on the body, which can include exercise and excitement.
In this study, we do not know what the men were doing when these higher levels were recorded and whether this differed from the time when the lower levels were recorded. They could have been exercising, watching films, or playing games, therefore affecting their results.
Although activity was reported to be "controlled" by monitoring, it was not clear whether this meant that activity was restricted in all periods, and the results were not adjusted to take activity into account.
Also, the other two stress-related hormones measured were not affected by sleep restriction or naps.
This study has not proven that naps improve the immune system, which has been reported in the media. IL-6 has a complex role in both stimulating and dampening the immune response in different circumstances.
Therefore, a one-off reading of IL-6 like this, without any other markers of the immune system, is difficult to interpret accurately.
What may have been a more relevant outcome is what effect a nap might have on concentration and the ability to think clearly and reason after a night of poor sleep.
This could have been achieved through psychometric testing, although the numbers would still have been small, limiting the power of the study to detect an effect.
Further limitations include the fact the study conditions did not mimic normal life – the participants had to stay in the sleep laboratory for three days at a time and were not allowed to drink tea, coffee or alcohol. They were also only sleep-deprived on one night. This means the results may not reflect what would happen in normal circumstances.
It is not clear what the men's normal daily schedules were like and whether having a break for three days from hectic life would have been less stressful, or conversely if being cooped up in a laboratory would have felt claustrophobic.
In conclusion, this study of just 11 men does not prove that naps successfully counteract the negative effects of losing a night's sleep on the immune system or symptoms of stress.
Many adults fall into bad habits when it comes to sleep, such as drinking alcohol before bedtime or overstimulating the mind late in the evening. You may need to improve your sleep hygiene – adopting better habits that will help promote better sleep. Read more about sleep hygiene.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.