"Absolutely, rest up" is more than sufficient in 99percent of cases
Internal monologue: "But wait, will it come off as impolite if my reply is this short? I better add something about how I'm sad to hear that they are sick. And maybe also something that I hope they will get better soon. Hmm... how do I say that without sounding like I expect them to be better soon-- that they can and should feel allowed to recover at their own pace? But, now it sounds as we don't need them at work-- I also want them to feel missed. Also, is there a risk they take 'rest up' wrong?, as if it is their fault they are sick because they haven't rested enough?-- I'd better soften up that formulation. Then, how do I start this email? 'Dear x,' seems too formal, maybe 'Hey,' -- no, that sounds like 'Hey listen up!'; maybe I'll just skip the greeting to make it feel more like a casual conversation. Do I still sign the email? With "Regards?", "Best regards?", "Sincerely?", "With wishes of swift recovery?" Should I also cut the email footer to make it seem less formal? What if they need to forward this to show that they have my permission? In that case the formal footer is probably useful.... etc. etc.
Crush both apples with the blunt side of the knife. Divide applesauce equally.