I'll try and dig out the letter I got from DVLA, I think they mention in there that if you have more than 3 hypos a year you are legally required to inform them. I don't remember if they mention anything about day or night time though.
When I was diagnosed, both my GP and Nurse told me that I HAD to inform DVLA but when I called them they said that you only have to inform them if you are on Insulin and sent me a letter in which the above information I talk about is mentioned. Can't remember it all exactly but I will dig it out and post it.