Half of fathers hardly ever or never get up in the night to tend to their crying babies, a survey shows.
Only a quarter wake up when the baby cries and a fifth wake up once the mother is already awake, Mother & Baby magazine's Sleep Survey suggests.
Quizzing 2,000 parents revealed 52% of dads sleep on, or at least pretend to.
The survey's commissioners said the lack of sleep and resentment this caused in new mothers often lead to relationship problems between couples.
"
It's shameful so many dads don't get up during the night because if parents work as a team they can get through this difficult period
"
Elena Dalrymple, editor of Mother & Baby magazine
Six out of 10 of the mothers said they resented their male partner for not getting up when the baby cried in the night.
Nearly three-quarters said lack of sleep had spoiled their love-life and most said they preferred sleep to sex.
Six out of 10 said having a child had put their relationship with their partner under "immense strain".
Mothers generally felt "fed up, exhausted and pulled in too many directions", with 94% thinking paid maternity leave should be for 12 months rather than the current six months.
Strain on relationships
Elena Dalrymple, editor of Mother & Baby magazine, said: "It may be logical for mums to get up more at night during their maternity leave, but once she returns to work it should be 50/50.
"It's shameful so many dads don't get up during the night because if parents work as a team they can get through this difficult period.
"Mums desperately need support and the obvious person to support them is dad.
She said parents-to-be often had no idea of the strain the lack of sleep a baby brings could have on relationships.
"
Maybe the partner can help at the weekend or empathise more and not come in and say 'what have you done all day?'
"
Alison Dawson, postnatal tutor for the National Childbirth Trust
"If you're only getting four hours sleep night after night and sometimes less, you'll most likely take your frustration and anger out on your partner.
"It's amazing so many relationships survive the onslaught of a baby, but sadly, some never recover."
The strains appeared to be compounded when both in the couple were in full-time work.
More than three-quarters of mothers who returned to work and more than half of fathers said a lack of sleep was affecting their ability to work.
Full-time job
Two-thirds of working mothers and fathers said bosses were not understanding.
Alison Dawson, postnatal tutor for the National Childbirth Trust who holds classes for new mums, said: "In my experience, the findings reflect what happens in life."
She said new parents were often surprised by how little sleep they were getting. She said realising and preparing for this by discussing it before the baby was could help relieve tension.
"If people can think 'it's only going to last for three or six months hopefully' and start to think 'this is only a temporary phase', that helps them get through the postnatal period."
She said having a supportive partner was extremely important.
"Maybe the partner can help at the weekend or empathise more and not come in and say 'what have you done all day?'
"That can be very difficult for a woman if you are used to working.
"Understanding that having a new baby is a full time job helps," she said.
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