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They all basically work the same for sleep tracking. The wrist worn trackers do it automatically, the clip-on One needs to be worn on your wrist and manually started / stopped. There's a FAQ on the sleep tracking that may help at https://help.fitbit.com/articles/en_US/Help_article/1314/
As they are all similar for sleep tracking then your choice may be driven by other features. There's a good comparison chart at www.fitbit.com/compare
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