Many new mothers suffer from extreme tiredness even four months after giving birth, prompting experts to warn they should not hurry back to work.
An Australian study found half of new mothers were still 'excessively sleepy' 18 weeks into motherhood.
The findings have 'significant implications' for decisions-makers about the length of maternity leave, the researchers said.
Dr Ashleigh Filtness, of Queensland University of Technology, studied the sleep patterns and tiredness in women after giving birth.
She found that despite new mothers recording stable night sleep times at 18 weeks, they continued to report being excessively tired.
The study, published in the journal PLOS ONE, followed 33 healthy new Australian mums who recorded their sleep patterns after giving birth in 15 minute increments during weeks six, 12 and 18.
Dr Filtness said: "Sleep disruption strongly influences daytime function, with sleepiness recognised as a risk-factor for people performing critical and dangerous tasks."
"Our findings bring into question whether four months parental leave is sufficient to ensure daytime sleepiness has diminished to a manageable level before returning to work."
And while new mothers were still waking on average twice a night to attend to their babies at six, 12 and 18 weeks - their total sleep time was about seven hours and 20 minutes.
Australian new mothers actually slept more than the average American one, who got six hours and 53 minutes.
Dr Filtness added: "While post-partum women experienced disturbed sleep, they didn't necessarily experience total reduced sleep time.
"What we found was that inevitably, new mothers will wake in the night to attend to their infant and the number of times they wake remains consistent during the first 18 post-partum weeks."
"Sleep disruption reduced over time and it appears this was driven by a reduction in the time it took for new mums to return to sleep, suggesting improved efficiency by mothers at settling their infant or the development of the infant's circadian rhythm."
She added the findings highlighted the importance of sleep quality as opposed to sleep quantity - especially during the first 12 weeks.
Referring back to the length of maternity leave a woman has, Dr Filtness said: "It is important when developing regulations for parental leave entitlements that policy makers take into account the high prevalence of excessive daytime sleepiness experienced by new mothers.
"With the birth of every baby the new mother must adjust to the demands of parenting and one aspect of that is to remain functional while experiencing potentially severe sleep disruption.
"To put this into context, the assessment tool used to determine new mums' sleepiness is also used by GPs to determine clinically relevant levels of sleepiness.
"If any other otherwise healthy person presented to a doctor with this degree of sleepiness they would likely have been offered advice regarding implications for daytime impairment including the impact on sustaining attention and decision making."
- Daily Mail