ABSTRACT
Diurnal Variation in Airways Caliber The spontaneous diurnal variation in airway caliber is fundamental for understanding the physiological measurements of lung function in relationship to work. There is a spontaneous diurnal rhythm (for instance, in peak flow) that can be demonstrated in the majority of the normal population, but is exaggerated in asthmatics (1). The lowest readings are usually around the time of waking; there is then an improvement for 6-8 hours followed by a subsequent decline until sleeping, with a further decline overnight. The trigger for the diurnal variation appears to be sleep stage, airway caliber being reduced during periods of rapid eye movement (REM) sleep, particularly during narrative dreams. Any reaction at work will be superimposed on this diurnal variation. The relationship between work and sleep can be substantially altered by shift work. Most day workers and workers on early shifts (for instance, 06:00-14:00) wake shortly before going to work. The first few hours at work will therefore coincide with the period of improving lung function. The effects of an immediate reaction is then to blunt the rise in peak flow rather than to cause any fall, as shown in Figure 9.1 (2). Workers
on afternoon shifts (for instance, 14:00-22:00) usually wake some time before going to work and go to sleep shortly after returning from work. This results in the occupational exposure, coinciding with the declining phase of lung function. Immediate reactions are usually more obvious on such shifts (Fig. 9.2). The patterns of sleep in night workers are highly variable and usually change between the days at work and the days away from work. The patterns of reaction can then be complex. The diurnal variation usually resets itself within 24 hours of a change in sleep pattern, unlike the diurnal variations of cortisol and adrenaline, which take a longer time to reset.
