Essential oils do not help you to actually sleep, rather, they work in conjunction with your endocrine excretions to help support each one.
Diffusing a calming oil during the day works with your cortisol excretions and helps you focus.
When used at night (when your body is producing melatonin and your cortisol has worn off), it will help your mind calm down and focus and then your body is more ready to fall asleep.