You should not feel guilty if you struggle to drag yourself out of bed for an early morning work-out, according to a new study that claims the best time to exercise is during the day.
Muscles operate according to their own circadium rhythm, meaning your body responds differently to exercise depending on the time of day, according to research by Chicago's Northwestern University.
The paper, published in the scientific journal Cell Metabolism, claims muscles are most efficient when you are most awake.
"Oxygen and the internal clock are doing a dance together inside muscle cells to produce energy, and the time of day determines how well that dance is synchronized," author Professor Dr Joseph Bass said in a statement.
"The capacity for a cell to perform its most important functions, to contract, will vary according to the time of day.
"We're not saying we can tell athletes when they should work out but in the future, perhaps, you may be able to take advantage of these insights to optimise muscle function."
Dr Bass found that when they turned off the "clock" in the muscle cells, they prevented the normal capacity of exercise to induce sugar consumption and generate lactic acid.
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These internal clocks can be manipulated with drugs and Dr Bass said the findings could have far-reaching consequences.
“In the future, we may discover new ways to manipulate the oxygen response of the cell by resetting the clock," he said.
"It’s also a critical step in understanding how to impact glucose metabolism in diabetes.”