The latest sunrise of the year is this week. Thank daylight saving time.

Commuters fumble for car keys in the chilly gloom. Teenagers wail in disbelief that the clock says “Get up!" while the sky says to stay under the covers.

The dreaded dark days of late December? No, this week.

Thanks to the extension of daylight saving time that went into effect in 2007, the latest sunrises of the year now occur in late October and early November. This year in Philadelphia, the latest one comes on Saturday, when the sun makes its laggardly appearance at 7:31 a.m.

“It’s bizarre,” said astronomer John Bochanski, an associate professor at Rider University in Lawrenceville, N.J.

Things return to normal the next morning with sunup at 6:32 a.m. (Remember to turn clocks back an hour.)

Fun bonus fact: Even without daylight saving, the latest sunrise still would not occur on Dec. 21, the winter solstice and darkest day of the year, but in early January. That part is not so bizarre, at least to an astronomer like Bochanski, but it requires some explanation.

But first, the human impact.

Dark mornings are not simply a matter of inconvenience. Teenagers in particular do not function well in the early hours, and jokes aside, it’s not due to laziness. It’s because their biological clocks prompt them to stay up later in the evening. The American Academy of Pediatrics has called for later school start times as a result, seeking to ease an epidemic of sleep deprivation and associated health consequences.

» READ MORE: Later school start times could address ‘crisis’ of sleep-deprived teens, Pa. report finds

Is the impact even worse during these dark mornings of extended daylight saving? Hard to say, as there has been little study of the issue, but these extra weeks of early morning gloom can hardly be a plus. The law postponed the end of daylight saving by one week in the fall and added three or four weeks at its beginning, in March.

On the flip side, the extension was enacted in part to allow more daylight at the end of the day for trick-or-treating — a valid concern, as traffic fatalities historically have been higher on Halloween. Candy merchants were in favor of the switch as well.

Center City resident Jake Taylor, who was walking his dog, Ella, before sunrise last week, said he does not mind the late sunrise.

“That morning time, especially when it’s dark out, I enjoy it, because the city’s not crazy yet,” said Taylor, co-owner of Bluebell Kitchens in Wayne.

Regardless of what period lawmakers choose to establish the artificial construct of daylight saving, there is no avoiding the fact that the days grow shorter with the waning year.

On Dec. 21 in Philadelphia this year, the sun rises at 7:19 a.m. and sets at a dreary 4:38 p.m., making for a day that lasts just nine hours and 19 minutes.

The number of hours of sunlight each day starts increasing after that. But sunrises continue to edge a bit later until early January because the true solar day is not exactly 24 hours long, said Rider’s Bochanski, chair of the university’s department of computer science and physics.

Defined as the time between when the sun appears in a certain position in the sky on one day and when it reaches the same position on the next, a solar day is a few seconds longer than 24 hours at this time of year because the Earth’s orbit is elliptical, he said. (At some other times of year, it is less than 24 hours.)

“You sort of have a little bit of drifting,” he said.

Chew on that when you are out trick-or-treating.

Or not. The forecast for Halloween this year looks like rain.