I wouldn't add a scalable sleep spell to my campaign It sounds too exploitable in combat. Something that would be continually cast to bring down opponents and robbing the sense of hard won victory from the players. Instead of that giant staggering on with its last few HP, still a deadly threat, the encounter becomes a throat-slitting ending over and over again. Sleep is a nice little low level spell for the generally fragile and under powered mage and this change threatens to make it something used with every combat. It sounds like a sugar-teat which is fine for an infant but gets Game of Thrones disgusting once the kid is old enough to be cast out of Sparta to fight with wolves over his dinner.