This Kentucky horse breeder had a filly that won every race in which she was entered. But as she got older she became very temperamental. He soon found that when he raced her in the evening, she would win handily, but when she raced during the day she would come in dead last. He consulted the top veterinarians and horse psychologists to no avail. He finally had to give up because it had become a real night mare.

Joke found on http://www.ahajokes.com