JAR OPS clearly states that controlled rest is allowed. An extract to that effect was actually removed from our Ops Manual a few years ago but crews must be totally comfortable that the regulator (UK CAA) recommends controlled rest to improve performance if required. It seems to be a bit of a taboo subject but it is normally management that are anti in-flight rest and of course - they never fly the line as much as the rest of us!
I have no hesitation in advising my F/O I may need a 'snooze' and I hope they would speak out if they were needing a 'snooze' as well! Providing it is done properly - it is no threat to safety!