The invention is directed to sensing techniques that can be executed by an implantable medical device (IMD). The sensing techniques exploit the fact that useful information can be sensed during the periods where the sympathetic and vagal tone balance changes in a patient's nervous system. This balance generally changes when a patient is falling asleep or waking up from sleep. In accordance with the invention, sensed information is recorded specifically during the times where a patient is either falling asleep or waking up. The IMD can be designed to sense or identify when a patient is falling asleep or waking up, and can record the useful sensed information specifically during those times.