Five in the morning. Always five in the morning.
It started during the first half of winter. Then, I could get to sleep well enough without too many layers of sheets, blankets or whatever for warmth, but by the very early morning it was freezing as all hell. The temperature always dropped significantly while I slept, and the sheer cold started forcing me to wake up every night at 5 AM, at which point I threw on as many layers of sheets, blankets or whatever as possible and went back to sleep. But by the time the nights started warming up again, waking up at that time had become part of my sleep schedule. Even when I stayed up for hours and resolved to sleep in until 12 PM or later to make up for lost sleep, I still woke up at 5 AM. Perhaps it was more psychological at that point; was I only waking up at that time because my body expected to be forced to?