We might get up at the crack of dawn for a jog to get in shape but we would get more benefit working up a sweat at lunchtime, a new study suggests.
And the findings could also lead to better diabetic treatments.
Muscles have circadian or body clocks that control exercise response and the time of day is important in getting the best out of a workout.
Oxygen and the internal clock "do a dance together" in muscle cells to make energy, scientists from Northwestern University found.
Muscle cells are more efficient during an organism's normal waking hours while all cells, including those in muscle, contain a clock that regulates how cells adapt to changes in the environment and activity across the 24-hour day.
Senior author Professor Dr Joseph Bass said: "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 synchronised
"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."
He added the research has implications beyond muscle cells because oxygen response is important in all cells.
In particular, the deprivation of oxygen is a key factor in heart attacks and in cancer, in which the depletion of oxygen curiously enables cancer cells to grow.
The study involved mice who are naturally nocturnal and found they adapted to exercise better at night.
They were exercised on a treadmill at different times of day, as well as in isolated muscle fibres in which the circadian clock was genetically mutated.
The scientists analysed mouse muscle tissues and muscle fibres for expression of genes that are important for exercise
This allowed them to determine the impact of deregulation of the circadian clock on muscle fibres in terms of how muscle processes fuel, like sugar and fat, when oxygen levels are low.
Prof Bass said: "When we manipulated the clock genetically, we noticed there were profound abnormalities in the muscle.
"That set us on a course to understand how the inner muscle clock is important in regulating how well the muscle cell can mobilise energy."
When mice exercised at night, their muscles are better at turning on genes to help them adapt to exercise
Since these genes also exist in humans, this suggested humans may also be able to respond better to exercise during the daytime.
The muscle clocks control the metabolic response by interacting with proteins called HIFs that change metabolism when oxygen concentrations get too low in order to allow muscle cells to continue to make energy.
Normally when we rest or do low-level exercise, our muscles consume oxygen to make energy.
Yet when we start to sprint or exercise strenuously, we consume oxygen faster and quickly run out.
That's when the dip in oxygen triggers HIFs and signals muscles to switch to sugar for energy, which in turn increases lactic acid.
Turning off the muscle clock prevented the normal capacity of exercise to induce sugar consumption and generation of lactic acid.
These findings suggest that better exercise capacity may be tied to specific times of day.
Prof Bass added: "In the future, we may discover new ways to manipulate the oxygen response of the cell by resetting the clock.
"If we can optimise muscle function it's also a critical step in understanding how to impact glucose metabolism in diabetes."
Diabetes is characterised by a failure of muscle to consume glucose, which in turn controls blood sugar levels.
Strengthening the muscle clock may provide a new way to eliminate excess glucose and treat diabetes.
The study was published in the journal Cell Metabolism.