Couples sleep better if they have sex beforehand.
Researchers have discovered that people nod off faster and get better quality sleep if they orgasm with a partner but solo sex acts do not have the same effect.
Getting tired out by the exertion of making love and a surge in sleep-promoting hormones play a part in the findings.
Study author Carlotta Oesterling got 159 people to fill out questionnaires and two-week diaries about their love lives and sleeping habits.
She discovered that if they had been intimate with their partner first, the average time taken to get to sleep fell from 21 minutes to 16 minutes.
Oesterling, a PhD Student at the University of Groningen in the Netherlands, said: "Orgasm is established to release oxytocin and prolactin - hormones that both influence sleep.
"The psychological effects of relationship satisfaction, loving and feeling loved, as well as having a sense of security, have also been shown to impact sleep."