As kids, it's drummed into us that to be selfish is wrong. Share your toys. Share your food. Share the airtime in conversations. Of course this is sensible guidance, since living with others in relative harmony means not keeping it all to yourself. The theory is that if you share with others, then they'll share with you too. However, being unselfish isn't the same as believing that it's somehow wrong to care for yourself, and to look after No. 1 at times.
When you travel by air, the safety demonstration tells you to put on your own oxygen mask before tending to the needs of others, and this clearly makes sense. If you're going to be of help to those around you, you must be strong yourself, so ensuring that you're well-rested, well-fed and well-adjusted isn't selfish. It's right and proper.