Sensing position, gyroscopy, motion accelerometers...

Hi all.
Never explored those sensors that register where is up, down, steady, motion, turning...
I suppose the toy quadcopters should be using something related.

How would you implement a device that (having a real time clock ?) will transmit on WiFi its movements ?
Like when a change of positioning happens, waking up to transmit the change that happened at what time ?
Powered by a 3V coin cell would be nice.
Thinking better, no time clock needed to be transmitted, the receiver can log that. Just sending Bluetooth the position changes to a smart phone that with some code will time-stamp and save the events.

To log a human body position/movement -say during sleep- Or what existing gadget could be re-purposed for such ?

You need an accelerometer, easily interfaced to an ESP8266/ESP32 for WiFi, or the ESP32 for WiFi or Bluetooth. I think there may well be a number of ready made modules that include everything you need, plus a few other sensors as well.

You might find this page helpful?:

You can get modules with accelerometers, compasses and other gizmo's built in just for this purpose.
I've built a few devices with the esp8266 and esp32, sending data over the net is simple, however datarate can be an issue, if your sending a batch of data every 20 seconds or so use json, but if you want a fairly quick continuous update then you'll need to setup a datastream.