Here ya go Hugh...

Sleep Tight meaning: Many years ago beds were made out of a wood frame with ropes stretched taut accross it on which the mattress would sit. The ropes would stretch and knots would loosen and the mattress would sag and become uncomfortable. So, fairly often, the ropes would have to be tightened so sleeping could be comfortable again. Hence the term 'sleep tight' became synonymous with sleeping comfortably and getting a good night's sleep. Now you know way more than you ever wanted to know about the phrase "sleep tight". lol