take some time. you may have to do it every day until your baby sees this as a routine.
you may have to make sure that everything that you do at day time is lovely and will be felt by the child. at night, while you are still in a good mood and you can still show your love to your child, make everything is quieter. your child will start to feel the difference and would also adjust to this type of scenario.
by doing this, you will be helping the body clock of your child to become more obvious. after some time, you will start to see him/her knowing to difference 