People who work night shifts are constantly fighting against all “internal clock” in their bodies. Quite often the clock tells them to sleep when their job requires them to remain fully awake. It’s no wonder that more accidents happen during night shifts than at any other time. Light therapy (光照治疗法. with a bright light box can help night-shift workers adjust their internal clock. However，many doctors recommend careful planning to help improve sleep patterns. For example，night-shift workers often find it difficult to sleep in the morning when they get off work because the body’s natural rhythm (节律. fights back，no matter how tired they are. Some experts recommend that night-shift workers schedule two smaller sleep periods---one in the morning after work， and another longer one in the afternoon，closer to when the body would naturally need to sleep. It’s also helpful to ask friends and family to cooperate by avoiding visits and phone calls during the times when you are sleeping.
Night-shift worker often find it difficult to sleep in the morning because _____.

A.their internal clock will not allow them to

B.they are often disturbed by morning visits

C.they are not trying hard enough to do so

D.they are too tired to go to sleep well