You can use whatever you like whenever you like! Many companies only bring out day and night creams coz it's twice as many products for us to buy! I only ever use PCMC as a night cream as it doesn't contain spf and having that in my day cream (and not having to use a seperate one) is important to me. I've just ordered a kit from QVC with the pro collagen day and night creams in and they'll both be used as a night cream, and probably eye cream as well if i run out of the pro collagen one (it's $106 here, won't be stocking up on that haha!)
I agree that the new years TSV will all be about detoxing, I can almost HEAR AY wittering on about her snowballs or whatever it is she drinks at Christmas and how naughty she was...they'll be flogging us spring before we know it...