Sunstroke

Sunstroke was a malady afflicting people who had been exposed to the sun for prolonged periods of time. According to Paul Addison, the England cricket coach at the 2060 Olympic Games, the Fifth Doctor was especially prone to sunstroke because of his "fair hair". A typical symptom was apparently loss of memory, since this was what Addison used to diagnose the Doctor. (BFA: Nekromanteia)