Also known in more pedantic circles as the cap of perfect repose:
This cap, when donned, will lull one into a deep, restful sleep lasting precisely eight hours, after which it will slip off, allowing one to awaken fully refreshed and alert. During the magical slumber, the wearer will be utterly oblivious to all sounds, smells, lights, jostlings, or other incidental stimuli occurring in the vicinity including earthquakes, hurricanes, meteor bombardments, clog-dancing, and other bothersome disasters. The slightest deliberate attempt to wake the sleeper, however, even if it is the merest whisper, will rouse the wearer instantly, causing the cap to fall off his or her head and cancelling the rejuvenatory effects of that slumber. To place the cap on a subject who is aware of the attempt and actively resisting, the instigator must make a successful "to hit" roll. The subject is not entitled to a saving throw, but can be roused quite easily by anyone attempting to do so.