If the temperature is above ~25 deg C (don't know what that is in caveman units) and I am not very careful about regulating my exertion level, after the ride I will get a nasty headache and usually feel nauseous. I have been experimenting with stopping and quickly drinking a slushy or other ice drink to intentionally give myself an 'ice cream headache' - my theory is I am directly icing my brain to reduce inflammation.