The human body has a circadian rhythm, the body’s own internal 24 hour clock. It tells the body when to sleep and when to be awake. The circadian rhythm of the body has been tuned to understand that it needs to sleep when it is dark and awaken when it is light. People who work night time shifts or rotating shifts need to sleep during the day. Their body’s circadian rhythm may not be able to adjust to their sleep-wake pattern. As a result, they are not able to get enough sleep during the day. This lack of sleep can cause drowsiness, irritability, fatigue etc. If this continues for a long period of time, shift work sleep disorders can result.

Not all people who work night shifts have this problem. Some people find it easier to work at night than during the day. People who work in early morning shifts from 4 AM or so may also have shift work sleep disorder. Similarly, people who switch between morning, evening and night shifts also have trouble sleeping at times. This is because rotating shifts disturbs the body’s sleep-wake pattern. Your internal clock may find it difficult to reset between sleeping at night and sleeping during the day.