Thursday January 15 2009
Not getting enough sleep affects the body's ability to fight off viruses
“Getting fewer than seven hours of sleep a night puts you on the fast track for a cold,” the Daily Mail reported. The newspaper was referring to a study which found that adults deprived of sleep were three times more likely to catch a cold than those who sleep for eight hours or more.
This study is based on the theory that sleep restores the immune system. The researchers interviewed volunteers about their sleep patterns over a two-week period, and then exposed them to a cold virus. They found that people whose sleep is typically interrupted (sleep inefficiency) are nearly six times more likely to catch a cold. This factor was true regardless of how long they slept.
Overall, this study was well-conducted and provides reliable evidence of a link between lack of sleep and increased susceptibility to colds. The exact nature of the link and the effectiveness of any related treatments to prevent colds need more study. Ideal duration of sleep may be seven to eight hours a night, but quality (sleep efficiency) also seems important.
Where did the story come from?
Dr Sheldon Cohen and colleagues from the Carnegie Mellon University in Pittsburgh in the US carried out the research. The work was funded by several grants to the Pittsburgh Mind-Body Center, including the National Heart, Lung, and Blood Institute and the National Institute of Allergy and Infectious Diseases. The study was published in the peer-reviewed journal Archives of Internal Medicine.
What kind of scientific study was this?
In this cohort study, the researchers studied 153 healthy men and women with an average age of 37 between 2000 and 2004. The researchers searched for a link between reported sleep patterns and susceptibility to developing a cold after all participants were exposed to the cold virus.
Previous research has suggested that people who sleep seven to eight hours a night have the lowest rates of heart disease. In this study, the researchers wanted to see if regularly getting a good night's sleep can aid immunity levels and, specifically, ward off a cold.
The researchers used an advertisement to recruit 78 men and 75 women for the experiment. The recruits were paid $800 to participate, and were studied in six groups. Anybody with a serious medical condition or who had undergone nose surgery was excluded.
The volunteers were then given a physical examination and asked routine questions about their height and weight, social background, alcohol and smoking habits. They also had blood tests that looked for pre-existing antibodies to the respiratory viruses that cause colds.
Over a two-week period, the volunteers were interviewed by phone about their sleeping habits. They were asked questions such as, “What time did you lie down to go to sleep?” and “Did you feel rested in the morning after sleep?”. The total time asleep and sleep scores were then calculated from these answers. These scores helped the researchers estimate the “sleep efficiency” of the volunteers, i.e. the percentage of time in bed actually spent sleeping.
Finally, the volunteers were put in “quarantine” for five days, isolating them from others who might have been carrying a virus. During the first 24 hours they had a nasal examination, nasal lavage (irrigation of the nasal cavity), and their mucus production measured. They were then given nasal drops containing a heavy dose of rhinovirus, which causes the common cold.
For the rest of the quarantine period, the volunteers reported any signs and symptoms of illness. The researchers assessed the volunteers’ daily nasal mucus production and how well the mucus cleared from their nasal passages. They also collected daily mucus samples and tested them to see if they contained the cold virus.
Twenty-eight days after exposure to the virus, blood samples were taken from each volunteer and tested to see if they had developed antibodies to fight the virus, indicating that they had caught a cold. The researchers defined “having a cold” as being infected with the virus (i.e. having cold virus in their mucus or producing antibodies to the virus). Having a cold was also defined either through self reported (subjective) symptoms of a cold, or through objective signs of a cold (i.e. high mucus production or poor mucus clearance).
The researchers analysed both the subjective and the objective measures of having a cold. They then adjusted their results (took into account) for 16 socioeconomic factors, plus other factors that had been recorded in the first interview.
What were the results of the study?
Over a third of the volunteers (35%) developed a cold according to objective measures, and 43% developed a cold according to subjective measures (self-reported symptoms).
Having a lower recorded sleep efficiency (spending more time in bed trying to get to sleep, or sleeping for a shorter period of time) were both associated with an increased risk of developing a cold (based on objective and subjective measures).
Volunteers who spent 92% or less of their time in bed actually asleep were five-and-a-half times more likely to become ill than those whose efficiency was above 98%. People who slept for less than seven hours a night were almost three times more likely to develop a cold than those who slept eight hours or more. The researchers made analyses that were adjusted for sleep efficiency when assessing the effect of sleep duration, and vice versa. They found that adjusting for sleep efficiency removed the effect of sleep duration, but not the other way around.
How rested a person felt after sleeping did not affect their risk of catching a cold.
What interpretations did the researchers draw from these results?
The researchers say that poorer sleep efficiency and shorter sleep duration in the weeks before exposure to a rhinovirus was “associated with lower resistance to illness”. They also say that sleep duration alone did not predict the association between sleep and illness. This suggests that of the two measures, sleep efficiency may be the more important link to catching a cold.
What does the NHS Knowledge Service make of this study?
It may not be surprising that measures of sleep predicted the risk of catching a cold when the virus was inserted into the volunteers’ noses. This study’s complexity lies in the measures chosen to monitor sleep habits, as well as the attempts made to find patterns of sleep that could explain this increased risk of catching a cold. Some points raised by the researchers and the newspaper commentators include:
- The study’s strength lies in the prospective nature of the study, in that volunteers were questioned before exposure to the virus and followed over time. This increases confidence in the results.
- The researchers report that sleep duration and sleep efficiency still had a significant effect after taking 16 different factors into account, including ethnicity. This increases the confidence that these other risks are not responsible for these results.
- Underlying stress is hard to measure and control for. Therefore it is not possible from this study alone to tell whether the colds are caused by stress linked to poor sleep, or caused by sleep disturbance itself. The fact that sleep efficiency was more strongly linked to developing a cold than sleep duration suggests that stress may play a part in the process.
- Self-reported sleep may be less exact than objectively monitored and recorded sleep. The authors acknowledge that this may introduce bias, but say it is unlikely to be a problem among healthy volunteers.
- The common cold can be caused by a variety of different viruses, but only the respiratory virus RV-39 was tested in this study. Although it is likely that other viruses would have similar results, this cannot be confirmed until separate studies are carried out.
Overall, this study was well-conducted and provides reliable evidence of a link between lack of sleep and increased susceptibility to colds. The exact nature of the link and the aspect of a sleep pattern responsible has yet to be identified. The effectiveness of any interventions that might help prevent colds by improving sleep also remains unknown.
Sir Muir Gray adds...
I have never worried about catching a cold, they are part of life.