If you're looking for a cheap, hackable device with basic sleep tracking, I highly recommend the Xiaomi Mi Band (1 or 2). The protocol has been reverse enginee

If you're looking for a cheap, hackable device with basic sleep tracking, I highly recommend the Xiaomi Mi Band (1 or 2). The protocol has been reverse engineered, so you can use a program like GadgetBridge to get all of your raw data out of the device.