You've only just started haven't you? It could be because your body is just getting used to not having so many calories to fuel it so is sleeping as no energy.
I got sleepy the first week and was in bed by 9pm most nights even though I usually dont go to bed until 1am.
I find now though that I go to bed early but wake up early. So typically bed at 11-12 and wake up at 7am.