The last time I was on a cruise ship which was 5 years ago I found that I actually slept better on the cruise ship the second night than I normally would at home.
At first it was kinda a weird feeling to be laying their in bed while the cruise ship was moving down the ocean slowly and I felt a rocking motion which kept me awake some at the first the first night.
But then the next night after I got used to the motion of the cruise ship I actually drifted off to sleep and slept better.
Most other people I've talked too also said they had a better time sleeping due to the motion of the cruise ship and fell asleep faster and slept better.
You can also take some sleeping medicines to help you sleep if you're having trouble sleeping on the cruise ship.