A University of Minnesota study has confirmed what South Washington County school officials were banking on when they agreed to push back high school start times in 2009: Students who get at least eight hours of sleep are better prepared to learn.

Performances on state standardized math tests rose at the district's high schools — as did average daily attendance — following the move from 7:35 to 8:35 a.m. starts, said university researcher Kyla Wahlstrom.

However, according to the study, there were no significant changes in state reading test results.

Wahlstrom presented the findings to school board members in December.

There was a point of concern for district leaders in the generally favorable report: 88.5 percent of students surveyed at South Washington County's three high schools reported having a cellphone in their bedrooms.

Simply activating the light on a phone "has the potential for significant disruption of sleep," Wahlstrom told board members.

South Washington County Superintendent Keith Jacobus said he believed parents should take note of how cellphones can affect sleep. It was perhaps best, he said, to be away from electronic devices for about an hour before bedtime.

For nearly 20 years, Wahlstrom has studied how school start times affect students. She has pushed for later start times for high schoolers, in particular, because she says the teenage brain develops differently than an adult or prepubescent brain.

For teens, the idea of going to bed early to wake up early doesn't work, Wahlstrom said, because their bodies don't experience a natural shift into sleep until 11 p.m. — no matter when the school start time might be.

So the later the start, the better the chance of having a student in class ready to go.

About 59 percent of South Washington County students surveyed said they now average eight or more hours of sleep. The average bedtime on school nights was 11:28 p.m. and the average waking time was 7:22 a.m., the study showed.

Board Chairman Ron Kath said that he expected the board to take the data into account in future budget deliberations. He noted that transportation costs were a significant part of a district's annual expenditures, and that changes in start times affect busing expenditures.

Last year, when the school board set out to trim $2 million from the 2013-14 district budget, members agreed to extend school bus boundaries for middle school and high school students.

The move to later high school start times in 2009-10 also led to middle school students beginning their day earlier. That decision still concerns Board Member Marsha Adou, who noted at last month's board meeting that middle schoolers were nearly teenagers themselves and so the same concerns about lack of sleep should apply.

Wahlstrom said that an upcoming study out of Jackson Hole, Wyo., should provide meaningful middle school data.

Jacobus added that the issue of students and sleep could be the subject of one of the Parent University conversations he has hosted since taking over as superintendent about a year and a half ago.