I'm just starting a spell of 4:15am starts, 5 days a week for 8 weeks.
It plays havoc for the running in my
Duathlon training but at least it means I get extra miles in cycling between my two job locations.
Curiously for me I find I'm more awake, certainly so far, because it ensures I go to bed earlier.
The downside is I'm paranoid about not waking up so despite the loud alarm on the clock radio I do tend to wake up from time to time. I'm sure that will improve once I get used to it.