Writing in the journal Jama Pediatrics, Lack and a team of researchers behind the study say while there is a common belief that eating solid food helps a baby to sleep better - with one NHS survey suggesting most mothers give their child food before five months - many sources of advice for new parents, including the NHS and the National Childbirth Trust, recommend that parents should wait until six months before introducing solids.
The findings contradict current recommendations by the Center for Disease Control and Prevention, which advises parents to move babies on to solid food at 6 months old.
Mothers appeared to benefit, too, as sleep problems defined by the parents were reported less frequently in the solids group.
But the new study, by experts at King's College London and St George's University of London, found that in fact solid food increased sleep time and reduced the number of times a baby woke in the night. The group was split in half, with one group consuming exclusively breastmilk for six months and the other gradually incorporating solid food into their diet along with breastmilk. At night, the amount they woke up dropped from twice to 1.74 times on average.
The NHS and World Health Organization now advise to wait until around six months before introducing solid foods, but these guidelines are now under review.
"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 results of the study showed that the babies that started eating solid foods sooner slept longer, woke up less, and had fewer sleeping problems than those that consumed exclusively breastmilk for that six month period.
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".
Parents have been told for years to delay introducing solid food, mainly to encourage mothers to continue breastfeeding for as long as possible, but also because scientists thought food introduced earlier could lead to allergies.
Mary Fewtrell, a professor and a nutrition lead at the United Kingdom -based Royal College of Pediatrics and Child Health (RCPCH) who was not involved in the study, commented: "These are interesting findings from a large randomized controlled trial".
A Food Standards Agency spokesman said: 'This further analysis. could be of interest to parents, however, there are limitations to the findings. "Parents should introduce solids at the pace their infant is comfortable with", he said.
'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.
"We expect to see updated recommendations on infant feeding in the not too distant future".
Soft fruits, like peach or melon, or baby rice or baby cereal mixed with your baby's usual milk.
Keep offering different foods.
The NHS Choices website claims this is a mistake - and solid foods will not make babies more likely to sleep through the night. As part of the study the team also looked the impact on other measures, including growth and sleep.
'We are encouraging all women to stick to existing advice to exclusively breastfeed for around the first six months of age.