For men who roll over and nod off rather than engage in pillow talk, it’s the perfect excuse.
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 behavior.’
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.
In 2005, research found the blood rush after climax depletes the muscles of energy-producing glycogen.
And because men typically have higher muscle density than women, they become more tired after sex.
Dr Neil Stanley, director of sleep at the University of Surrey, said that men were not completely at fault.
‘As frustrating as it is for most women that their male partners just roll over and fall asleep after sex, men aren’t entirely to blame,’ he said.
‘Humans are the only animals in which sleep and sex are linked and while often seen as just a poor excuse, there are scientific reasons why men feel tired after sex.’
The survey showed the majority of men did feel more relaxed straight after sex.
Eighty per cent of men said they felt able to drift off without any problems after making love, compared with just 46 percent of women.
They were amongst 10,000 adults surveyed by organizers of The Vitality Show, Europe’s largest health and beauty exhibition.
The survey also found 48 per cent of men had actually fallen asleep during the act itself.
And while scientists might have rescued men from a telling off for sleeping straight after orgasm, it seems perhaps understandable that the latest research won’t do the same for those who doze before reaching climax.