A method is provided for measuring a wake up indicator, where the wake up indicator gives the likelihood of a user waking up undesirably. It is based on the knowledge that waking up during deep NREM sleep is not desirable due to sleep inertia and on the recognition that it is also undesirable to wake up during REM sleep due to atonia. A sleep inertia estimation is determined to estimate if the user is in NREM sleep and an atonia estimation is determined to estimate if the user is in REM sleep. The wake up indicator is determined from the sleep inertia estimation and the atonia estimation. The wake up indicator thus may be used as an indicator for the time when it is suitable to wake up the user in a way which avoids arousal from a deep sleep state or from REM sleep, during which atonia may arise.
A METHOD FOR DETERMINING THE RISK OF A USER WAKING UP IN AN UNDESIRABLE STATE