It usually means you haven't slept for long enough or you're catching up on a sleep debt. When you disrupt your sleep cycle or you don't get enough sleep, your body needs time to catch up. In some cases, this can take 4-5 days. Make sure you spend the next week going to bed at the same time, waking up at the same time, and getting 8-9 hours of sleep each night. You should find it much easier to wake up in the morning.