Because we would get hours to fall asleep on nap days, less overall sleep, unpredictable wake ups... just a mess. FWIW, I would never have believed the Groclock would have worked either. I think what makes the difference for loads of kids is that it's not mum or dad telling them 'it's the middle of the night', it's a 'third party' if that makes sense. We told Stan that if he looks at the clock and sees the moon, that means mummy and daddy are sleeping and he needs to lie down in his bed. (We rehearsed stretching, rolling over, grabbing lovey). It wasn't easy. The first night he just screamed from the moment he woke up until the sun showed up on the clock. We didn't even go to him (mummy and daddy are sleeping), but called out to him periodically that it is night time, mummy and daddy are asleep. The second night he screamed for a long time (can't remember) but did get back into bed (didn't go back to sleep) a few minutes before 'morning'. I think by the fourth morning we didn't hear him any more - if he woke up he just did like you and I would do - look at his clock, see it's night time, went back to sleep. It struck me, when he wakes at 5am, he has no way of knowing if it's night or morning so why wouldn't he get up and start the day - he's awake, so why wouldn't it be morning, yk? Anyway, bottom line is, if I would have known in advance how well it would work, I would have paid £200 instead of the £20 it cost. And it mitigated the effects of OT, because I knew he would sleep in the morning until a reasonable time, and if I needed to put him to bed particularly early or late for any reason, I could slightly adjust the time on the clock to take that into account and make sure he'd always (or at least, usually), get enough sleep.