Lung function varies throughout the day, being much lower at night. There may be a difference of more than 15% between the FEV 1 (Forced Expiratory Volume in 1 second) at night and during the day. While such fluctuations are normal, they are much more marked in individuals with asthma.
Individuals who experience night-time asthma symptoms have what is termed ‘nocturnal asthma’. Some of them may sleep even while having mild symptoms such as cough, while others may wake due to symptoms but then rapidly return to sleep. Still others may have difficulty sleeping due to asthma symptoms. The intensity of symptoms will depend on the degree of control of asthma as well as exposure to triggers. Triggers in the bedroom may be a cause of disrupted sleep. Exposure to triggers during the day may result in a late phase reaction that occurs anywhere from 4 to 6 hours later. The late phase can last for up to 24 hours, unlike the initial phase that is due to airway smooth muscle spasm, and is the result of the inflammatory cascade and tends to be more severe and last longer.
Nocturnal symptoms in asthma have a physiological basis. A review by Haxhiu1 showed that in sleep, while airway conductivity is lower, an increase in cholinergic output causes heightened bronchoconstrictive responses. In patients without asthma, this change passes un-noticed, but in patients with asthma, these alterations in lung function during sleep cause symptoms and frequent awakenings. They decrease the quality of life and increase mortality.
Sleep2 is associated with reduced lung function and increased airway resistance so that peak flow reading are lowest around 4 a.m. Further, at night mucociliary clearance is decreased while mediator levels rise. Histamine levels peak at around 4 a.m. Cortisol levels are lowest at 11 p.m. and peak around 7 a.m. Levels of epinephrine are lower while vagal tone increases. GERD may stimulate vagal receptors and thus increase bronchoconstriction. It is not surprising then that nocturnal asthma symptoms are related to circadian rhythms which regulate mediator levels, inflammatory cells, hormone levels, and cholinergic tone. See Table 1.
The mechanisms put forward for nocturnal asthma are:
- airway cooling
- allergen exposure
- gastroesophageal reflux
- increased tissue inflammation
- decreased plasma epinephrine
- decreased plasma cortisol
- increased circulating eosinophils
- increased cholinergic tone
- polymorphisms of the beta adrenergic receptor
Symptoms worsen with snoring, obstructive sleep apnea (OSA) and gastroesophageal reflux disease (GERD).
Ballard3 suggested that other potential mechanisms that are involved in nocturnal worsening of asthma include:
- supine posture
- changes in both the sympathetic and parasympathetic systems
- alterations in the sympathetic and parasympathetic balance
- reduction of lung volumes during sleep
- intrapulmonary pooling of blood, and
- normal upper airway narrowing during sleep.
Nocturnal asthma symptoms should be taken seriously. Any symptoms that disrupt sleep have an effect on daytime functioning; further, sleep disruption has long-lasting effects if left untreated. Thus it behooves the asthma educator or health care professional to consistently inquire of individuals with asthma as to whether or not they have nocturnal symptoms and how well they sleep.
- Haxhiu MA, Rust CF et al. CNS determinants of sleep-related worsening of airway functions: implications for nocturnal asthma. Respir Physiol Neurobiol. 2006;151(1):1-30
- Skloot GS. Nocturnal asthma: mechanisms and management. Mt. Sinai J Med 2002; 69(3): 140-47
- Ballard RD. Sleep, respiratory physiology, and nocturnal asthma. Chronobiol Int. 1999; 16(5): 565-80