If it's informal, as if you're talking to a friend or peer, then
you would say:
Descansa.
If it's formal, as if you're talking to a grandparent, or
acquaintance, then you would say:
Descanse.
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I would add to the above answer thar the "a lot" factor is
missing. So I would say "Descansa mucho" or "Descanse mucho".