You skipped the section where the measurement of total sleep for most devices was found to be more than adequate. Detecting light versus deep sleep accurately

You skipped the section where the measurement of total sleep for most devices was found to be more than adequate. Detecting light versus deep sleep accurately with wristwatches or devices under the mattresses might not be possible. Especially not when accuracy is supposed to be comparable to multiple electrodes places directly on the head to measure brainwaves.