New mothers are being urged to be cautious about returning to work too quickly, after a recent study suggests that one in two are still excessively sleepy four months after giving birth.

Researchers from the Queensland University of Technology in Australia studied the sleep patterns and tiredness of postpartum mums and found despite new mums recording stable night sleep times at 18 weeks, they continued to report being excessively tired.

"Sleep disruption strongly influences daytime function, with sleepiness recognized as a risk-factor for people performing critical and dangerous tasks," researcher Ashleigh Filtness said in a statement.

For the study, researchers followed 33 healthy new mums who recorded their postpartum sleep patterns in 15 minute increments during weeks 6, 12 and 18.

Filtness said the study has significant implications for decisions-makers about when women should return to work, with current government paid parental leave entitlements ceasing at 18 weeks.

"This brings into question whether four months parental leave is sufficient to ensure daytime sleepiness has diminished to a manageable level before returning to work," she 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."

Filtness said the study also found while new mums were still waking on average twice a night to attend to their babies at 6, 12 and 18 weeks -- their total sleep time was about 7 hours and 20 minutes.

She said Australian new mothers actually slept more than the average American worker (6h 53mins).

These findings highlight the importance of sleep quality as opposed to sleep quantity, especially during the first 12 weeks.

"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," Filtness said.

The findings were recently published in PLOS One.