The study found that infants in the group which had solids introduced early slept longer and woke less frequently than those infants that exclusively breastfeed to around six months of age.
Researchers from the United Kingdom and USA looked at data collected as part of a clinical trial exploring whether early introduction of certain foods could reduce the chance of an infant developing an allergy to them.
For the latest United Kingdom research, the parents of half the children were encouraged to feed their babies solids, such as white fish or wheat, before six months, while the other half were told to stick to breast milk alone until that time.
By five months, the babies who had started eating solids earlier were sleeping longer than those whose mothers were instructed to breastfeed exclusively for six months.
The researchers from King's College, London, and the University of London admitted it was possible that mothers giving their babies solids may have responded to their questions in a more positive manner, having expected a positive effect, since many parents already believe that the practice encourages better sleep.
At the six month mark, both groups of children were eating solid foods.
"When it comes to the recommendations for mothers and their children, I think it's still important to try to aim for exclusive breastfeeding for the first six months", he added.
But the new study suggests that advice is flawed - and suggests babies do better if solid food is given earlier, alongside breastmilk.
But he added: 'For healthy, full-term babies who are feeding well at the breast there are no good reasons to delay the feeding of solids, as long as breast feeding continues'.
Brown urged caution, noting that no difference in waking was seen until after five months, despite one group being introduced to solids from three months, and that self-report of infant sleep by exhausted parents was unlikely to be precise.
They also woke less frequently.
The findings provide some solid data to back up the long-held belief that feeding infants solid food helps them sleep better, Dr Jae Kim, a neonatologist at of the University of California San Diego and the Radey Children's Hospital of San Diego, told Reuters Health in a phone interview.
A Food Standards Agency spokesman said: 'This further analysis. could be of interest to parents, however, there are limitations to the findings.
Professor Mary Fewtrell of the Royal College of Paediatrics and Child Health said: 'These are interesting findings from a large randomised controlled trial.
'However, the evidence base for the existing advice on exclusive breastfeeding is over ten years old, and is now being reviewed in the United Kingdom by the Scientific Advisory Committee on Nutrition and in the EU by the European Food Safety Authority.
Writing in the journal Jama Pediatrics, co-author of the research Gideon Lack acknowledged that it's generally advised parents wait until six months to introduce solids.
'We expect to see updated recommendations on infant feeding in the not too distant future'.