“Evening men”, who go to bed and get up late, have more sexual partners and more extra-pair matings than “morning men”, who get up early and go to bed early. Evening people are often nicknamed "Owls", and morning people "Larks".

This is related to testosterone: evening types have a higher testosterone level. Does evening type signal good genes? To test this, we want to assess immunocompetence and sperm quality as surrogates of good genes and compare evening with morning types.