(Inside Science TV) -- By day, Curtis Snyder's twins, Sam and Lucy, are all smiles, but by night, it's another story.
"They don’t want to start the bedtime process most days," said Snyder.
Their problem is a common one. About 25 percent of toddlers have problems settling down to go to sleep at bedtime.
"Lucy, for example, has a hundred different excuses," Snyder said.
Now, a new study by sleep researchers at the University of Colorado Boulder found that bedtime battles could happen because the bed times that parents choose for their children is out of sync with the kids' biological clocks.
"The body knows when it’s time for sleep, and so for adults we get to choose what time that is, but for young kids, parents have to choose," said Monique LeBourgeois, one of the study's researchers.
The body knows that it's time to hit the sack when a hormone called melatonin begins to rise in the evening hours. Melatonin signals to the body that it is time for bed by causing drowsiness and lowering body temperature. Putting kids to bed before their melatonin levels are high enough makes it harder to fall asleep. For some, it can take at least an hour -- a sign that bedtime is not in sync with their internal biological clocks.
"Some kids had a melatonin onset at about 6:30 in the evening, whereas some children didn't have an increase in melatonin until about 9 or 9:30," explained LeBourgeois.
Scientists suggest reducing light at home and avoiding electronic devices in the evening. Parents also need to be flexible in adjusting bedtimes.
"We try to make it as close to 8:00 as possible ... obviously life happens and sometimes that doesn't work," said Snyder.
Researchers say that sleep problems in early childhood are predictive of emotional and behavioral problems later in life as well as poor cognitive function that can negatively affect them as they grow.
Parents should be aware of the change in season and how that might affect their child's sleep cycle and bed time.