In addition to the melatonin thing, sunlight helps us to metabolize Vitamin D. My ped actually wanted me to put Nat on baby vitamins from the get-go to help prevent a D deficiency, but we're outside regularly so I don't do it, but the ped did say that lots of babies do have a deficiency of D, which also means a calcium deficiency since I guess you need the D to absorb calcium. So I would say YES, being outside is GOOD for ALL of us - UNLESS the air quality is bad, in which case it's probably a toss-up, since you can have bad air inside OR out. :?