Researchers don't know for sure why autistic children have problems with sleep, but they have several theories. The first has to do with social cues. People know when it's time to go to sleep at night, thanks to the normal cycles of light and dark and the body's circadian rhythms. But they also use social cues. For example, children may see their siblings getting ready for bed. Children with autism, who often have difficulty communicating, may misinterpret or fail to understand these cues.
(This is one small reason why I do not like people at my house after 5 because my children need a lot more time than the average person to know and realize that it is time to settle down for bed.)
Another theory has to do with the hormone melatonin, which normally helps regulate sleep-wake cycles. To make melatonin, the body needs an amino acid called tryptophan, which research has found to be either higher or lower than normal in children with autism. Typically, melatonin levels rise in response to darkness (at night) and dip during the daylight hours. Studies have shown that some children with autism don't release melatonin at the correct times of day. Instead, they have high levels of melatonin during the daytime and lower levels at night.
Another reason children with autism may have trouble falling asleep or awaken in the middle of the night could be an increased sensitivity to outside stimuli, such as touch or sound. While most kids continue to sleep soundly while their mother opens the bedroom door or tucks in the covers, a child with autism might wake up abruptly. (This happens all the time! Yet another reason why I am so particular about sounds, lights, and people in my house. My husband and others have thought I was crazy so many times when I would try to explain that even the tv in the living room could very easily be waking him up at night. Trust me, when you go through what I do, you want to make sure there is nothing going on in your house after Trenton is asleep! Once he is asleep, everything and everybody else needs to go too! When most kids can hear a noise and then go back to sleep, it just doesn't work that way for kids with autism.)
Anxiety is another possible condition that could adversely affect sleep. Children with autism tend to test higher than other children for anxiety.