The description of the command spell doesn't list the effects of a "Sleep" command, but it does say:
Some typical commands and their effects follow. You might issue a
command other than one described here. If you do so, the DM determines
how the target behaves. If the target can't follow your command, the
spell ends.
If a DM allowed it, I imagine a "Sleep" command would be functionally equivalent to the "Grovel" option, which is listed in the spell description:
Grovel. The target falls prone and then ends its turn.
However, this effect would only last for that one turn; command doesn't cause a creature to instantly fall asleep and stay that way. The creature might try to lay down to sleep for that one turn, but nothing would cause it to sleep immediately; since the spell's duration is only 1 round, the creature could just get up on the turn after that.
Rules designer Jeremy Crawford unofficially addressed a similar question regarding the suggestion spell in a pair of tweets from December 2017:
if I use suggestions to tell someone to “go to sleep” do I have an 8 hour sleep spell?
The suggestion spell allows you to compel someone to pursue a course of activity. You could, for example, tell them to go to sleep. That doesn't knock them out; it causes them to try to go to sleep naturally. It isn't magical slumber.
Also, there's no guarantee they can actually sleep. E.g. telling someone to go to sleep in the middle of a battle should fail automatically.
Given how hard it often is for me to go to sleep, casting suggestion on me and telling me to go to sleep would usually result in nothing at all!
This ruling on suggestion seems to roughly correspond to my interpretation of using a "Sleep" command with the command spell.
Suggestion at least has a duration of "Concentration, up to 8 hours" (ending "when the subject finishes what it was asked to do" if it can be completed within that time), which at least allows for the possibility that the target will succeed in falling asleep naturally. In contrast, even if casting command and telling the target to "Sleep" works, it will have no greater effect than that of the existing "Grovel" option in the command spell's description.
