I have tried sleeping at 2am everyday. I have no problem waking up at 7am to get my girls ready for school.
Recently, I tried dragging it to 3am. No problem waking up latest by 7.30am.
Then I wanted to test my limit. I slept at 4.45am! I woke up the next day around 9am (it was a weekend) with runny nose and swollen piles! Even after I take a 3 hours nap in the afternoon, my body still cannot recover.
So, my limit is only until 3am.