It always resets at 1AM (Midnight when the clocks go back).
I work lates (Until 10PM most nights), so I often don't get to bed until around 3AM in the morning, as I have tea/evening time after work. As such I can take advantage of these offers when they are working. I've found when I am up during the day the offers don't always work.
There seems to be only a small window of a few hours (Probably between 1AM and 4AM) where the optimum period of claiming exists.