The bare minimum of sleep needed to live, not just thrive, is 4 hours per 24-hour period. Seven to 9 hours of sleep are needed for health, renewal, learning, and memory. Disruption of the sleep cycle from shift work creates problems for the quality and quantity of sleep. Shift workers average only about 5 or 6 hours of sleep. These disruptions usually contribute to such symptoms as shallow and/or fitful sleep, headaches, difficulty in concentrating, and upset stomach. Shift workers typically have weaker immune systems and are more likely to have emotional problems and experience higher rates of divorce and social handicaps. Shift workers can enhance their sleep periods by making sure the bedroom is as dark as possible or by wearing a sleep mask. Other sleep aids are exercise and yoga. Shift workers should not eat or drink food and beverages near bedtime, as sleep is likely to be disrupted by having to go to the bathroom. Losing sleep night after night creates a cumulative sleep deficit, which in turn causes serious health problems and even death.