buy an electronic egg timer
.. at the time of those tales ... the word electronic hadn't been coined .. but we all had one mechanical device or another to achieve the same end .. problem was remembering to set it prior to nodding off .. actually the main problem was trying to make sure that at least one of the 2/3 folk involved stayed awake .. can recall coming back from NZ (with a nav = 4 crew) after a mongrel trip for sleep ... I woke up going through FL200 on climb ... and had to force myself to stay awake as none of the others showed any interest in waking up ... OK, that makes two tales at my expense .. the others are all improper so I'll keep them to myself ...