Sleep is only worth it up to CR 1
Here is a table of the expected hp from sleep and for monsters on average using those in the Monster ManualMonster Manual, or using the guidelines given in the DMG for creating monsters of a given challenge rating, including the number of monsters you could put to sleep based on them:
| CR | Sleep hp | MM hp | DMG hp | Sleep/MM | Sleep/DMG |
|---|---|---|---|---|---|
| 0 | 22.5 | 3.5 | 3.5 | 6.4 | 6.4 |
| 1/8 | 22.5 | 8.2 | 21.0 | 2.7 | 1.1 |
| 1/4 | 22.5 | 14.8 | 42.5 | 1.5 | 0.5 |
| 1/2 | 22.5 | 19.7 | 60.0 | 1.1 | 0.4 |
| 1 | 22.5 | 28.2 | 78.0 | 0.8 | 0.3 |
By the numbers
As can be seen, already by CR 1, the average expected monster hit points outpace what sleep can put to sleep. You will not catch up at higher levels with upcasting -- you get to add 9 points of sleep for every spell level or two player levels, while the monster hit points increase on average by 13 per challenge rating in the MM (26 per spell level), or by more than 15 by DMG (30 per spell level).
As the question is defined, even on the first level sleep would not be worth it. That of course is nonsense, as you encounter a lot of smaller groups of monsters that are of CR 1/8 or 1/4 at that level. So, it does not make sense to look at this from a player level perspective, it is better to look at this from a monster hp and numbers perspective.
The issue is not primarily that you only face enemies with higher hit point totals at higher levels, the issue is that, as encounter XP values increase, you face larger numbers of enemies with the same hit point totals as before. And while removing four kobolds from a group of six is a powerful move, removing four kobolds from a group of sixty is a much weaker use of your single action for the round.
Play experience
In my own experience, you continue to encounter CR 1/8 to CR 1/2 monsters well into tier two play, and sleep may still useful as a cheap way to deal with them. However, at these levels, such monsters tend to come in larger groups, so you want to be able to remove all of them in the area, not just a few. Keeping a preparation slot for sleep has increasing opportunity costs in this context.
After entering tier two, you also can fireball and kill those monsters outright. A fireball can be expected to remove all monsters with less than 14 hp, no matter ifwhether they save or not. I found myself no longer preparing sleep, even when dealing with low-CR monster mobs, somewhere around level 7 or so.
