A CRJ-200 flight crew experienced a momentary deviation from their cleared GS intercept altitude when the autopilot intercepted a false glide slope and climbed in response. Distractions and possible fatigue were cited as factors in the Captain's delayed response and return to their clearance.

Narrative

As we approached Glide Slope (GS) intercept I looked away briefly to consult the approach plate. The First Officer made a comment that directed my attention back to the instruments; at which time I saw the GS capture and the aircraft [start to] climb. There was also an occasional; brief red GS flag. Apparently a weak and fluctuating GS indication had caused the autopilot to capture early. By the time I reacted and disconnected the autopilot the GS had begun to move back to its correct indication and the aircraft was following it but we had climbed to about 7;400 FT. During this time the First Officer's side displayed the correct GS indication without any flags. We reported the irregularity to ATC. They queried other aircraft but no one else reported any problem. After assuming manual control for a moment the GS signal returned to normal. I called for the autopilot again and the rest of the approach proceeded normally. ATC said nothing about our altitude; and there was no TCAS indication. No apparent problem was caused. I entered the GS receiver discrepancy in the logbook. Factors included a high workload; IMC weather; an unusually strong headwind and a duty period that followed an 8-hour reduced-rest overnight. Error: Self-distraction; momentary inattention to basic instrument scan.Possible solution include: Avoid complacency. Avoid putting too much trust in the automation. I obviously chose the wrong moment to glance away; otherwise I know I would have reacted much quicker. I should have asked the First Officer to look up the detail I needed to confirm. Finally; the company should avoid scheduling reduced rest overnights. After a typical delay the night before we ended up with less than eight hours at the hotel itself. Five to six hours of sleep is just not enough for me. Even with 21 years of seniority on the -200 it is no longer possible for me to bid around these schedules. I don't know for sure whether that was a factor in today's event; but I do know these overnights pose an [undeniable] safety threat.