Measures to cut the risk of cot death must be taken for day-time naps as well as night-time sleeps, expert say.
Babies should be put to sleep on their back to reduce cot death risk
Researchers said this includes both the advice that babies should sleep on their backs and that they should be in the same room as their parents.
They found three-quarters of babies who died during the day were sleeping in a room where an adult was not present.
The study, published in the International Journal of Epidemiology, studied 1,625 children.
Around 300 babies under the age of one die each year in the UK from cot death, or sudden infant death syndrome.
The latest research, carried out by experts from Bristol University, Newcastle's Royal Victoria Infirmary and the Nuffield Institute for Health, was conducted over three years and included 325 infants in the UK who died and 1,300 of a similar age who did not.
The overwhelming majority of deaths - 83% - occurred at night-time, but of those that happened during the day, 75% were when babies were left in rooms unattended.
The study, partly funded by the Foundation for the Study of Infant Deaths, found that one in four of the babies left unattended had bedclothes over their head, compared to one in 10 where a parent was present.
The results also showed that death could happen extremely quickly.
Among the deaths that happened during the day, 38% were observed to be alive 30 minutes before they were found dead and 9% 10 minutes before they were found.
Report co-author Peter Fleming said: "I think what this shows is that we cannot ignore what has traditionally been done.
"In the past and in other cultures it is unheard of to leave babies on their own, it is something people in western countries should get back to."
And Joyce Epstein, director of the Foundation for the Study of Infant Deaths, said: "Parents have long been advised of the importance of sleeping babies on their back and not letting their head get covered by bedclothes, and this study shows that the advice needs to be followed day and night.
"For day-time naps, we suggest keeping your baby nearby in a carrycot or in a playpen while you go about your daily chores or, if possible, having a nap whenever your baby naps."