I went down with a cold last week, the first day I was up at 2am all bunged up, coughing, and struggling to breathe. As I was sleepily sipping on my hot lemon and honey I remembered that I'm not alone - a lot of people have breathing difficulties at night. In fact, I'm really lucky that I do not have asthma, as most asthmatics have a lot of difficulties breathing during the night. A survey of 7729 asthmatics in the 1980s showed that 94% had woken up in the night at least once in that month with difficulty breathing .
Many asthmatics have to take preventative medication before bedtime
(c) Luke Morgan 2011
What causes this nocturnal asthma? Is it caused by our sleeping environment, or is there something innate about our body clock that intensifies breathing difficulties at night?
Our sleeping environment can make things worse. Allergens on our pillows (or teddy bears) can cause asthma attacks, even up to four days after contact. Then there's our position when we're sleeping, lying flat compresses our airways making it harder to breathe. Even cold bedroom temperatures have been known to trigger asthma attacks.
However, asthma is a textbook example of how the time of day affects our physiology. There are daily changes in many body rhythms that make breathing between the hours of midnight and 8am much harder. Two important hormones involved in healthy airway maintenance are cortisol and adrenaline.
During the day adrenaline relaxes the muscles around the airway and cortisol reduces swelling, making breathing easier. At night, our body releases lower levels of both of these hormones, causing airway muscles to constrict and swell. This makes breathing at night harder.
Daily changes in cortisol and adrenaline in healthy individuals at rest, adapted from Sheer, 2010 
The implications of this hormone change in normal people causes a detectable, but low, amplitude change in the day/night variation of airway function. However, in asthmatics the the rhythm in airway function has a greater amplitude, making them more vulnerable to breathing difficulties at night .
One way to measure our lung capacity is to record peak flow. The higher a person’s peak flow the better their lungs are. One study in 1980 looked at the peak flow of normal and asthmatic sufferers throughout the day and night . In the afternoon and early evening the peak flow was the highest in both normal and asthmatics. However, the asthmatic sufferers had far lower peak flow measurements during the night: a 50% difference in their best and lowest peak flow compared to only 8% in a non-asthmatic.
Asthma does not occur randomly over a 24 hour period, it is much more likely to attack during the night in most sufferers. There is still a lot to be investigated about the causes and types of asthma attacks that occur during the night. However, if you are constantly bothered by nocturnal asthma attacks, speak with your GP. By ensuring your asthma medication protects you during the night, you can avoid attacks and get a better night’s sleep.
 M. Turner-Warwick, “Epidemiology of nocturnal asthma,” The American Journal of Medicine, vol. 85, 1988, pp. 6-8.
 F. a J.L. Scheer, K. Hu, H. Evoniuk, E.E. Kelly, A. Malhotra, M.F. Hilton, and S. a Shea, “Impact of the human circadian system, exercise, and their interaction on cardiovascular function.,” Proceedings of the National Academy of Sciences of the United States of America, vol. 107, Nov. 2010, pp. 20541-20546.
 M. Smolensky and L. Lamberg, The body clock guide to better health, Holt Paperback, 2000
 M.R. Hetzel and T.J. Clark, “Comparison of normal and asthmatic circadian rhythms in peak expiratory flow rate.,” Thorax, vol. 35, Oct. 1980, pp. 732-8.