I went to bed at 1h30, and set the alarm so that it rings at 9 (after a 7.5 hour core sleep). Anyway, I woke at 6h30 by myself, after completing 4 sleep cycles), and wasn't tired at all. And for the first time, I remembered a dream (plenty of biphasic sleepers say they experience an increase in dream remembering), so I guess it's a pretty good sign.
It's 10 PM, and I'm starting to get really tired. I couldn't take a nap, because I did paintball all afternoon, and now I can't stop myself from yawning every 10 minutes... I'll try and get a 7.5 hour core sleep tonight to try and compensate; I hope it'll work out.