Do Sleep Clocks work? Which one is best?

OK Mummy's I'm thinking about getting a Gro Clock or similar before I completely lose my sanity! My 2-nearly-3 year old sleeps really well, goes down at 7 and sleeps all night but he's been waking up earlier and earlier and he is now at 5 am. I'm still up a couple of times a night feeding my 11 month old daughter and when he wakes and yells out that "It's morning time Mummy!" at 5 it wakes the whole family! So my question is do you use a clock, which one and do they actually work? TIA from a rather sleep deprived Mumma!