Fell asleep, refreshed. Is a nap good every day or just when exhausted?

Depends. There's at least one study (arch intern med. 2007;167:296-301) suggesting that a daily nap decreases death from heart disease. Realize that a siesta is somewhat cultural so your employer might not take so kindly to you "sleeping on the job". More importantly, be sure you're not taking a nap to make up for poor sleep from night before, eg sleep apnea, restless leg syndrome, etc.