WEDNESDAY, Nov. 30 (HealthDay News) -- As many parents can attest, a rough night may follow when their baby has been to the doctor for their first shots, due to increased fussiness or fever from the immunizations. But a new study suggests that the time of day that the shots are given may make a difference in both sleep and immune response.

Research has shown that immunizations "take hold" more strongly when an infant has a long, deep sleep afterward, which is why some parents give acetaminophen (Tylenol) proactively.

According to the new study, published online Nov. 28 and in the December print issue of Pediatrics, infants may sleep more soundly when their shots are given after 1:30 p.m., regardless of whether they have been given any medication.

In the study of 70 infants who were getting their first series of vaccines at around 2 months of age, mothers either were told to give their infants a premeasured acetaminophen dose before the shot and every four hours thereafter, or to adhere to standard care. Some doctors gave infants a dose of acetaminophen as part of usual care while others told parents to administer the medication if their infant developed a fever in response to the shots.

Babies slept about 70 minutes longer in the first 24 hours after the shots, particularly if their immunizations were administered after 1:30 p.m. This change was unrelated to their weight, age or whether they were given acetaminophen. Most infants also showed the expected increase in body temperature, which indicates that the vaccine is taking effect. Those who had higher temperatures after the vaccine also got more sleep, the study showed.

More research is needed to back up these findings before any firm recommendation can be made about the best time to give infants their shots, the study authors noted.

"Based on what we currently know about sleep and the immune system, parents should try to help their babies to sleep well in the days before as well as after immunizations," said study author Linda Franck, a pediatric nurse at the University of California, San Francisco. "What we are learning about sleep and immune response to vaccines is just another reason for parents to learn how to help their baby sleep well."

Commenting on the study, Dr. Carol Baker, a professor of pediatrics, molecular virology and microbiology at Baylor College of Medicine in Houston, said that the decision about whether or not to give acetaminophen proactively before an immunization is best left up to parents and their pediatricians.

"Any concerned parent should discuss this issue with their pediatrician," Baker said. "Some parents are not comfortable with a 2-month-old having any fever or giving an infant medication."

A baby is more likely to develop a fever after their first set of shots then thereafter, Baker noted. "The study is small and was done at a single center; we would need a huge study with multiple sites to say 'you should have your child immunized in the afternoon,'" she added.

This may be easier said than done. "In terms of a busy practice and busy parents, we have enough trouble getting kids immunized and protected in time without throwing in this time-of-day thing," Baker pointed out.

More information

Visit the U.S. Centers for Disease Control and Prevention for more on child immunizations.