All airplanes in a now-defunct Swiss airline used to have "egg-timer" type wind-up clocks installed somewhere on the pedestal...
Now, at nap-time, you would set this alarm at something like 15 to 25 minutes, thus ensuring your own wake-up if the buddy supposed to stay awake did not. Simple and effective.
We did not like cabin crew checking into the cockpit all too often during the night, so as not to wake the resting pilot during his "mini sleep"
The reason we did not do more than about 25 minutes of sleep at the time are well-known. Drop into a full sleep cycle with REM-phase, and your awareness is more likely than not shot for a good while even after the wake up... short naps however work magic on those pesky red-eyes...
Both pilots nodding off during cruise happens... it's not nice, but it happens. So ensuring your own wake-up when napping just makes sense, to save major embarassment, or worse.