Researchers said the increased responsibilities of parenthood were responsible for sleepless nights, long after the demands of feeding and nappy changing had ended.
Before having children, men and women got just over seven hours' sleep a night, on average, the study found.
Lead researcher Dr Sakari Lemola, from the Department of Psychology at the University of Warwick, said: "While having children is a major source of joy for most parents it is possible that increased demands and responsibilities associated with the role as a parent lead to shorter sleep and decreased sleep quality even up to six years after birth of the first child."
Researchers blamed "changes in duties, strains, and worries related to the parental role" for long-term sleep loss.
The study also found that levels of sleep satisfaction remained lower, six years after having children.
The study, published in the journal Sleep, analysed data on 4,659 parents who had a child between 2008 and 2015.
Participants were followed for more than six years and reported on their sleep during yearly interviews.
The study concludes that for first-time mothers, childbirth is "the most significantly sleep-altering life event" during their adulthood.
The research showed that breastfeeding mothers got the least sleep, on average getting 14 minutes less slumber than those relying on bottle-feeding.
Dr Lemola added: "Women tend to experience more sleep disruption than men after the birth of a child reflecting that mothers are still more often in the role of the primary caregiver than fathers."
The study, published in the journal Sleep, analysed data on 4,659 parents who had a child between 2008 and 2015.
Participants were followed for more than six years and reported on their sleep during yearly interviews.
The study concludes that for first-time mothers, childbirth is "the most significantly sleep-altering life event" during their adulthood.
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