A few studies have been published concerning this topic. For example this one: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1330995/
To my knowledge they all had somewhat similar results:
Seven solitary subjects, and two groups of four, spent from 5 to 13
days in an isolation unit without knowledge of time. Three solitary
subjects and one group of ...
When you are forced to wake up, you've interrupted your body's circadian rhythm. If your body is used to waking up at particular time and you're woken before that time, you'll still feel very tired. Because to your body, you're still meant to be sleeping.