Sleeping twice a day is the key of success. Even though I had a 7.5 hour core sleep (midnight -> 7:30 AM), I was really tired all day long because of intense physical activites friday and saturday, combined with the lack of sleep yesterday afternoon, and today's family trip. Devin Reams said that having an extra cycle during the core sleep if you skip a nap will make you recover, but apparently it's not my case (or maybe it's because I still didn't fully adapt). It's 5:00 PM now, and I think I'll go and take and nap right away (instead of 6:30) because I'm really tired, and it will give me extra free time tonight (plenty of homework).
The nap was wonderful. I'm not tired anymore, and because it was successfull I'm going back to a 4.5 + 1.5 schedule. I stayed alert the rest of the evening, and was very inspired when I started writing an essay due next week; biphasic sleep makes you more productive :)