Why am I sleepy all of a sudden?

Before, I used to sleep around 12 or 1 am and woke up until 7 am. But recently, I have been getting very sleepy right after I eat dinner at 6 pm and then I fall asleep at 7pm. And I wake up in the middle of the night pick up things in my room and fall asleep again. Why am I suddenly getting so sleepy around the late afternoon? Before I used to have plenty of energy to last until 12 am.