Smoking is also linked with daytime sleepiness. The researchers suggest smokers might be using the stimulant effect of nicotine to self-treat their drowsiness.

Bixler's team studied a random sample of more than 16,000 men and women ranging in age from 20 to 100 years, nearly nine per cent of whom reported excessive daytime sleepiness. A random sample of more than 1,700 people received further evaluation in a sleep laboratory.

Depression was the most significant risk factor for excessive daytime sleepiness, followed by body mass index (a measure of obesity), age, typical sleep duration, diabetes, smoking and finally sleep apnea, a condition in which sleep is interrupted by repeated pauses in breathing.