Sleep duration and caffeine consumption in a French middle-aged working population

To explore the association between sleep duration and daily caffeine intake in a working population. Caffeine acutely disrupts sleep in the laboratory, but the inter-relations between sleep and caffeine intake in daily life are ill-known.

Questionnaire and diary based survey of 1498 persons from the GAZEL cohort of employees of the National Electricity and Gas Company (EDF-GDF) working in various locations in the Paris and South-West France areas. We analyzed total sleep time, our primary measure, and time in bed, both by sleep logs. We assessed daily intake of caffeine, consumption of alcohol and tobacco, use of hypnotics, and daytime somnolence, all by questionnaire.

Multiple linear regression analysis did not find a significant relationship between total sleep time and daily caffeine intake less than 8 cups of coffee per day, after controlling for age, gender, alcohol intake, smoking status, and use of hypnotics. By contrast, time in bed was reduced as caffeine intake increased (beta=-0.125; P<0.001). Higher caffeine intake was not related to a higher daytime somnolence.

Despite the well-known acute effects of caffeine on sleep, habitual use of up to 7 cups of coffee (or 600 mg of caffeine equivalent) per day was not associated with decreased duration of sleep.