Tummy time is a colloquialism used to encourage parents to ensure that their infant children spend time in the prone position while awake and supervised.
In 1992, the American Academy of Pediatrics recommended babies sleep on their backs to prevent Sudden Infant Death Syndrome (SIDS). Although the rate of SIDS decreased by 50% since the Safe to Sleep campaign started in 1994, an unintended consequence was that babies missed out on the twelve or so hours they used to spend in the prone position and there was a sharp increase in skull deformations in infants. Along with tummy time, rotating the direction infants lie in their crib as well as avoiding too much time in car seats, carriers, and bouncers are behaviors recommended to alleviate the associated risks of infants sleeping in a supine position.
Scientific research
Since 1998 there have been several studies published which report that infants placed to sleep in the supine position lag in motor skills, social skills, and cognitive ability development when compared to infants who sleep in the prone position. In a 1998 article entitled "Effects of Sleep Position on Infant Motor Development" by Davis, Moon, Sachs, and Ottolini, the authors state "We found that sleep position significantly impacts early motor development." The prone (stomach) sleeping infants in this study slept an average of 225.2 hours (8.3%) more in their first 6 months of life than the supine (back) sleeping infants.
In the 1998 article entitled "Does the Supine Sleeping Position Have Any Adverse Effects on the Child? II. Development in the First 18 Months" by Dewey, Fleming, Golding, and the ALSPAC Study Team the objective of the study was "To assess whether the recommendations that infants sleep supine could have adverse consequences on their motor and mental development." They used the Denver Developmental Screening Test (DDST) and studied infants at 6 and 18 months. According to the study, at 6 months of age, the infants who were placed to sleep in the prone position had statistically significant higher social skills scores, gross motor scores, and total development scores than those infants who were put to sleep in the supine position. In the 2005 article entitled "Influence of supine sleep positioning on early motor milestone acquisition" by Majnemer and Barr they used the Alberta Infant Motor Scale Scores (AIMS Scores) to analyze the impact of infant sleep position. They reported that "Typically developing infants who were sleep-positioned in supine had delayed motor development by age 6 months, and this was significantly associated with limited exposure to awake prone positioning." But, the authors also note that awake prone (stomach) positioning is associated with prone (stomach) sleeping. No studies have been conducted which compare supine sleeping infants who have regular awake prone positioning to prone sleeping infants who have regular awake prone positioning.
Placing infants on their stomachs while they are awake has been recommended to offset the motor skills delays associated with the back sleep position but positioning the infant on their stomach while awake will not impact the amount of slow wave sleep since tummy time only occurs when an infant is awake.