Diurnal variation of asthma.

We have studied possible aetiological factors in the large diurnal variation in peak expiratory flow rate seen in some asthmatics. This phenomenon occurred in both atopic and non-atopic patients. It could not be attributed to recumbency in bed at night or to allergy to the house dust mite in bedding. Circadian variation in asthmatic shift workers was intimately related to sleep and virtually independent of solar time. Treatment tends to lose its effect as it is discontinued during sleep and this increases the apparent diurnal variation in many patients. Excessive circadian variation may be an important factor in sudden asthma deaths. The importance of effective nocturnal medication is stressed.