This paper aims at acquiring knowledge about the quality of sleep of adult and elderly psychiatric patients who receive clinical or outpatient nursing care, and identifying key factors in perceiving a sleep problem. To do so, a sample of 1699 psychiatric patients were asked whether they perceived a sleep problem and were invited to fill in the Pittsburgh Sleep Quality Index (PSQI) and additional questions. Five hundred and sixty (33%) questionnaires were returned. As a result, we find that 36% of the patients perceived a sleep problem, while the PSQI assessed 66% of the sample as being 'bad sleepers'. Forty-nine per cent of the respondents used sleep medication one or more times a week. Five items of the PSQI were shown to be predictors of a perceived sleep problem. Four of these are insomnia symptoms, while the fifth is the use of sleep medication. Moreover, the patients who used sleep medication most scored significantly worse on all PSQI components than patients who used sleep medication less than once a week. In conclusion, many psychiatric patients perceive a sleep problem and all nurses could be confronted not only with the night-time consequences of this, but also with daytime consequences. Therefore, sleep problems must not be viewed as an isolated problem but must be seen in relation with social functioning.