What sensor to fix whether a person is lying on the bed or not?

Question

Sensors

Scales immediately come to mind - put under the leg. But high accuracy is not needed, and strain gauges for a suitable load are very expensive. Springs, etc. are not suitable, because everything must be firmly fixed. The project is an alarm clock that won't let me sleep at the wrong time. Perhaps the scales are not the only option. For example, to measure the temperature, but this, it seems, is also not cheap. Tell me what can be used?

Answer 1

Look towards the car sensors, which record that the passenger is sitting, but not wearing his seat belt. Strain gauges may work.

Answer 2

Brothers Grimm, "The Princess and the Pea" on the contrary)))
The temperature sensor is too inert - the bed cools down for a long time.
The carcass is wet and salty, a radio signal from 1 GHz (wavelength 30 cm) and higher is perfectly reflected from it.
Side effect: if you pissed, it is useless to get up, the alarm clock will not shut up.
Ultrasound - without this defect, but strongly extinguished in featherbeds.