You’re not selfish if you take the afternoon off because you’re tired or headachey or cranky or whatever.
On the contrary: you’re selfish if you keep working while you feel like that. Because then your coworkers have to deal with you when you’re not at your best, all to save you the discomfort of saying you aren’t feeling it today.
If you’re feeling less than 70%, don’t be selfish. Tell your team you need to take off for a bit.