Babies given solid food plus breast milk from three months sleep better than those who are solely breastfed, according to a new study.
Official advice is to breastfeed exclusively for the first six months of life.
Experts say women should still heed this recommendation, although it is under review.
In the study, in JAMA Pediatrics, giving solids earlier than six months had benefits for mum and baby.
The babies had fewer sleep problems and mothers reported improved quality of life.
The study, by King's College, London, and St George's, University of London, surveyed 1,303 three-month-olds, and divided them into two groups.
One group was solely breastfed for six months, the other group was given solid foods in addition to breast milk from the age of three months.
Parents then filled in online questionnaires every month until their baby was 12 months old, and then every 3 months until they were three years old.
The study showed that infants in the group who ate solids as well as breast milk slept longer, woke less frequently and had far fewer serious sleep problems than those who were exclusively breastfed until about six months.
The NHS and World Health Organization currently advise to wait until around six months before introducing solid foods, but these guidelines are currently under review.
Despite the official advice, 75% of British mothers introduced solid food before five months, with a quarter (26%) citing infant night time waking as the reason for their decision, according to the Infant Feeding Survey of 2010.
Prof Gideon Lack from King's College, London, said: "The results of this research support the widely held parental view that early introduction of solids improves sleep.
"While the official guidance is that starting solid foods won't make babies more likely to sleep through the night, this study suggests that this advice needs to be re-examined in light of the evidence we have gathered."
The differences between the groups peaked at six months, with the group fed solids early sleeping for a quarter of an hour longer per night (almost two hours longer per week) and waking less frequently - 1.74 times a night rather than twice a night.
Co-author of the study Dr Michael Perkin, from St George's, University of London, pointed out that small differences generated large benefits for parents. "Given that infant sleep directly affects parental quality of life, even a small improvement can have important benefits."