Yep, common problem especially for long duration events. All three times I did the Tour Divide bikepacking race I had issues for several days falling asleep after 14-16 hours in the saddle. After the first event, where I can vividly remember the first night 'clock watching' until I fell asleep at 5:30 am only to get up at 7, I researched the issue, and found that the stress hormone cortisol is the likely culprit. Basically I just had to endure several days until lack of sleep overcomes cortisol.
As evidence, I submit "Morning #3" :
