Controlled rest in the flight deck is vital to us maintaining alerness. On longer sectors, I try to plan a 10 or 20 minutes 'eye rest' an hour or so before TOD. Just shutting the eyes and having a short break increases my alertness greatly. As far as short sectors, I use to fly in the canadian arctic, 10 sectors in a 13:50 minute duty day... helping with all aspects of paperwork, loading and unloading. Half way through the day, we would be sure to tilt the head back (longer sector... 1.5 hours) for a bit and take a good look at the inside of our eyelids... for 5 to 30 minutes... sometimes 45. Made for a much more productive and safe second half of the day.
To say that this practice is unsafe is crazy. As a pax, I would feel much safer and secure knowing that the guys in the pointy end were fresh and well rested as they drove it down to the sometimes hostile airport environment.
my 2 cents worth