This spell is more powerful than almost any other spell in 5e, and thus should not be allowed in its current state
With the exception of wish, this might be the most powerful spell in the game if implemented the way you have it right now. If you had to pick a spell level right here and now with no modifications make it 9th level because that is the highest that 5e goes so far. However, I think this spell goes beyond the power of basically every other spell and thus should be changed or removed from the game. Some reasons for this are as follows:
It only has one save to literally defeat an enemy completely
Imagine your PCs fighting against your BBEG. Each person can cast this spell every turn with no downsides. If there is a bard in the party they can even make it more likely that the creature fails. If the BBEG fails and is alone the PCs won the battle. After the BBEG is unconscious, the PCs can cast it continually to make BBEG keep failing the save and essentially unconscious forever.
Compare this to the 8th level spell power word stun:
A creature you can see is stunned if it has 150 hit points or less. Otherwise, nothing happens. The target can make a Constitution save at the end of its turns, being freed on a success.
This spell has multiple limitations to it including a HP limit, and a save at the end of each turn to wake up and it is still a very powerful 8th level spell. Your spell is currently much much more powerful than this.
It has a short casting time
Imprisonment is another spell (9th level) with a somewhat similar effect, but arguably this spell is even more powerful than it.
As one of its effects imprisonment has slumber which says:
The target falls asleep and can't be awoken. The special component for this version of the spell consists of rare soporific herbs.
However, it has several limiting factors. The main one is that imprisonment has a 1 minute casting time. This makes it very difficult to cast in battle (that is 10 full rounds of doing nothing but casting the spell) and thus less likely to be used to completely shut down an opponent in the midst of a fight. The spell also requires very expensive and rare components to cast and also has multiple conditions that can end the spell.
It requires a very specific type of healing to reverse
In the example above, even if BBEG is not alone they need allies with very specific restoration magic to help (and a PC with counterspell could even mess up this way to help). If none of their allies have restoration magic, they might as well be alone because there is no way for them to help. In 5e similar spells and effects generally have other ways to reverse them going from minor (using an action to wake something up) up to major (dispel magic, wish). Combined with only one save the fact that the only way to reverse this is specific magic makes this extremely potent.
Even non-spellcasters can cast it
In its current state this "spell" is not really even a spell since it can be done by non-spellcasters. This makes this spell incredibly powerful beyond even any other listed spell. You have to take an entire feat just to get a couple low level spells. Gaining access to this uber-spell without any kind of requirements is insanely overpowered.
It can be spammed infinitely
Casting this spell doesn't take any kind of resource (like a spell slot) or limits (like times per day), so there is absolutely no reason for someone not to try to cast this each turn. Why try and do anything else when you have such a powerful ability with no cost?
This ability needs some serious fixing
As written, this ability is out of the realm of any other feature in 5e. Some of the bullits above are not overpowered on their own, but when you take them all together you get a feature that is incredibly powerful.
You need to make some serious changes to it as soon as possible to correct the issues I have laid out above. Even with limits, this ability is likely going to be incredibly powerful.
As it is now it now this ability is literally game-breakingly powerful.