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.
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 first group was fed only with breast milk for six months, and the second group was fed with solid food, together with the breast milk.
It's been proven that the babies from the second group, who were breastfed and were given solid food have slept for longer and have woken up less frequently - overall, their sleep problems were nearly non-existent, in comparison with those babies who were only fed with breast milk.
However, most mothers in Britain already ignore this advice, with 75 per cent introducing solids before five months and 26 per cent saying they did so to stop their babies becoming hungry overnight.
"Given that infant sleep directly affects parental quality of life, even a small improvement can have important benefits", says Dr. Michael Perkin, a co-author of the study from St George's, University of London.
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.
And the better babies sleep, the better their parents' quality of life and mental health. The difference peaked at six months of age, with the early introduction group sleeping an average of almost 17 minutes longer, and persisted after the infants' first birthday.
Thai football team rescue celebrated in the Dutch media
The rescued boys had not been identified out of respect for the families whose sons were still trapped, officials had said. Fifa boss Gianni Infantino had invited the boys' Wild Boars football team to Sunday's World Cup final last week.
They were also far less likely to suffer serious sleep disorders.
But they said it was unlikely that the bias would have persisted beyond six months.
"There is no clear physiological reason why introducing solids foods early would help a baby sleep, especially not for the very small amounts parents were instructed to give in this trial", she said. Babies who sleep more may wind up consuming less breast milk, he added.
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.
It comes after a milestone study by Professor Lack's team two years ago found allergies are actually less likely if certain foods - particularly peanut butter - are given to babies from an early life.
"However, the evidence base for the existing advice on exclusive breastfeeding is over 10 years old, and is now being reviewed in the United Kingdom by the Scientific Advisory Committee on Nutrition".
'We are encouraging all women to stick to existing advice to exclusively breastfeed for around the first six months of age.
"We expect to see updated recommendations on infant feeding in the not too distant future".