Tracking your sleep patterns can make for interesting reading, especially if you are regularly disturbed in the night by children, noise, insomnia, changing shift patterns or any number of other reasons. This device will give you a good idea of your sleep quality and quantity. It is not perfect as if you lie very still when trying to get to sleep, it assumes you are asleep which can skew the results.
The wrist band is like a large watch and has to be quite snug on the wrist to work properly. It has occasionally resulted in pins and needles in my hand, but I think that's just when I've been sleeping in a funny position! It's simple to charge - just plug it in to its base in the morning and let it charge up through the day ready for use at night. The connector can be a little fiddly until you get used to it.
As for a silent alarm clock, it wakes you up by vibrating against your wrist. It always seems to have done the job quite well, and is certainly a nicer way to wake up than having a normal beeping alarm clock going off next to your head. So far I've always set a second alarm clock a few minutes after the lark is due to wake me just in case it doesn't work if I'm especially tired, but I have to admit I haven't needed it yet. It's not quite silent, as the vibrating makes a feint buzzing noise, so if your partner is a light sleeper, they may well hear it though it is a lot less instrusive than a loud beeping sound.