A number of studies have highlighted the increasing incidence and financial cost of sleep-related disorders in the general population, but little research has been carried out on the impact of subjective daytime somnolence on health status. The existence of a survey of the health of employees of the French National Gas and Electricity Board has allowed us to investigate this question and measure the link between subjective daytime somnolence and sickness absenteeism, used here as a general health indicator. In order to evaluate the quality of sleep over the previous 3 months, a questionnaire was given to each participant. The association between subjective daytime somnolence and absence as a result of sickness was explored using the data of sickness absenteeism provided by the company’s social security department during a 12-month follow-up period. Of our 1105 subjects, 6.7% reported severe subjective daytime somnolence of 3 days or more a week and 30% of our study population had at least one spell of sickness absence during the 12-month period of follow-up. A strong association was observed between subjective daytime somnolence and sickness absence, which remained significant even after adjustment for potential confounding variables (age, sex, employment grade, sleep symptoms and self-reported diseases). The odds-ratio for absence as a result of sickness during the follow-up period associated with subjective daytime somnolence of 3 days or more a week was 2.2 (95% CI: 1.3–3.8). Employees suffering from severe subjective daytime somnolence lose more working days for health reasons than their more alert colleagues. This may have long-term implications for employees’ health.