It's basically the standard monk's schedule. Might be a shock for late sleepers used to eating and snacking throughout the day. Enough of a shock to set off some people. The Thais have an idiom for it which translates to "allergic to the robe". It happens to new monks sometimes. I saw it happen to one new monk in the group I ordained with. A few days after ordination I saw him running and yelling that the senior monks were trying to kill his family.

At least at the Dharma Drum Mountain retreats I've been to, it's true they have restrictions on what to eat and when. For example, garlic is contra indicated, offensive smell and supposedly makes you randy. Eating dinner is contraindicated, offered, but you'll notice the monks not eating it and just having tea or something. It's considered a medicine meal for the sick. Personally, I do feel more restful and ready for sleep at the end of the retreat days without dinner, but I do see how people not used to it could consider it food deprivation.

Going "hey non early bird, enforced early bird schedule NOW" is a shock to the system. It doesn't matter if it's normal to a completely different person.