The US has switched to daylight saving time, or summer time, three weeks earlier than usual to cut fuel consumption and help the environment.
Switching to summer time early should help cut energy consumption
At 0200 EST (0700 GMT) clocks moved forward by an hour, shifting an hour of daylight from morning to evening.
Summer time will last until 4 November, a week later than in previous years.
The extra four weeks are expected to help cut energy consumption, as demand falls for electricity in the evening if it is still light.
The measure was signed into law two years ago as part of the Energy Policy Act which aims to encourage new energy technologies.
Representatives Edward Markey and Fred Upton, who sponsored the amendment to the original bill, said it was expected to save $4.4bn in energy bills by 2020 and avoid the need to build more than three large electric power plants.
They said it also would save 279 billion cubic feet of natural gas, and avoid nearly 10.8 million metric tons of carbon emissions.
"The change in the beginning of daylight saving time is just one step towards making our country more efficient in its usage of energy and conscious of our environment," Mr Markey said on Wednesday.
"Not only will Americans have more daylight at their disposal for four additional weeks in the year, but we will also see wide energy saving, less crime, fewer traffic fatalities, more recreation time and increased economic activity.
"Ultimately, daylight saving just brings a smile to everybody's faces."
Critics of the measure say the early switch may potentially lead to computer failures and cause minor headaches such as electronic calendars being out of synch, leading to missed appointments.
Canada also advances its clocks this weekend, with all provinces moving forward an hour, except Saskatchewan which does not observe Daylight Saving Time.