DAFNE BI advice

So currently half way through my DAFNE course. Some great info am enjoying it but one thing I can't understand... My BI I was taking split at 8am and 8pm. DAFNE recommend to take it on rising and retiring ie if I go to bed at 11pm take it then and again when I wake (as long as there is 7hours between). I can't understand the reasoning behind this and my dsn hadn't read up on the reasoning so couldn't help. Can anyone shed some light? Id prefer to stick to the times I currently take it but obviously not if there is a better rational for this xx