The circadian clock controls
the sleeping cycle. This internal mechanism is particularly driven by the
hormone melatonin which is produced by the body when is little light, mostly
when night falls. This hormone tells the brain to rest, and so makes you fell asleep.
The night shift that Hanna is taking is messing with her melatonin production
and hence her circadian clock making very difficult to her to conciliate sleep
the nights that she does not have to work.
