Have you ever wondered why men doze off after they get some? They really do need to sleep after sex because the male brain is designed to switch off at that point, scientists claim.
They scanned men's brains before and during orgasm, and found that the cerebral cortex – or 'thinking' area – shuts down.
Straight afterwards, two other areas, the cingulate cortex and amygdala, tell the rest of the brain to deactivate from sexual desire.
This is accompanied by a surge of chemicals such as oxytocin and serotonin, which can have a powerful sleep-inducing effect, according to the researchers.
Neuroscientist Serge Stoleru, who leads a group at the French medical research council Inserm, said: 'These experiments give us the first hints as to what happens in the brain during orgasm. After men have an orgasm they usually experience a refractory period when they cannot be aroused.
'For women it seems to be different. They don't seem to have such a strong refractory period and may be asking for more when their partners just want a rest.'
One of the chemicals they release is prolactin, which is linked to the feeling of sexual satisfaction. Previous research has found men who are deficient in prolactin are less tired afterwards.
Dr Stoleru, whose research is published in the journal Neuroscience and Biobehavioral Reviews, said: 'The human brain is involved in all the successive steps of sexual behaviour.'
It may be a relief to some women to know there is a biological explanation for the roll-over-and-sleep effect. The study also showed that the brain switched off almost all sexual desire after orgasm.
And as well as this, men have a further reason for feeling drowsy: their brains become flooded with sleep-inducing chemicals such as serotonin after they have had sex.
Earlier this year psychologists at the University of Michigan in the US found women whose partners nod off are left feeling insecure and craving affection.
Other studies have also suggested that men aren't to blame for wanting to take a well-earned break after sex.
So there it is ladies.....