One in two new mums are still extremely tired at four months, a study shows, raising questions about when mums should return to work.

New mums are being urged to be cautious about returning to work too soon after a study found one in two are still excessively tired four months after giving birth.

Dr Ashleigh Filtness, of Queensland University of Technology, says her study has significant implications for decisions-makers about when new mums should return to work, with current government paid parental leave entitlements ceasing at 18 weeks.

“If you think about how anyone who is tired has problems with things like concentration, decision making and attention, then you have to question whether that person is ready to be in the workforce,” she said.

Dr Filtness followed 33 healthy Queensland mums through the first 18 weeks after they gave birth, to gain a unique insight into postnatal recovery.

They were each issued a diary in which they recorded every episode of sleep and wake each day during weeks six, 12 and 18.

They also filled in questionnaires based on the Epworth Sleepiness Scale, which is used to determine the level of daytime sleepiness.

Dr Filtness found 50 per cent of participants were still excessively sleepy at four months, waking on average twice a night to attend to their babies.

“They were all showing some signs of sleepiness but half of them were excessively tired.”

Dr Filtness said if a “normal” healthy person went to their doctor with the same degree of tiredness as a new mum, they would likely be offered advice regarding sleepiness and implications for driving and daytime impairment.

“Soon-to-be mums should be aware of the importance of their own sleep and consider how they are going to preserve their own sleep during the first few months of caring for a baby.”

The study was published in the journal PLOS ONE.