A pair of "rush hours" every day rapidly change the way tissues throughout the body work, scientists have discovered.
The animal study, in Proceedings of the National Academy of Sciences, monitored the function of cells, in 12 tissues, through the day.
It found large shifts in activity just before dawn and dusk.
Experts said the findings could help time medication to hit sweet-spots in the body clock.
The body's internal clock is known to drive huge changes - it alters alertness, mood, physical strength and even the risk of a heart attack in a daily rhythm.
A team at the University of Pennsylvania investigated the impact of the time of day on the way DNA functions in experiments on mice.
Every two hours they looked at samples from the kidney, liver, lung, adrenal gland, aorta, brainstem, cerebellum, brown fat, white fat, heart, hypothalamus, lung and skeletal muscle.
They showed that 43% of genes, sections of DNA, involved in protein manufacture altered their activity throughout the day.
Different genes had different activity patterns in different tissues so the research team conservatively estimate that more than half of genes would show daily fluctuations if every tissues type was sampled.
The liver was the most dynamic with 3,186 genes showing a daily pattern compared with just 642 in the hypothalamus.
Two major windows of activity were observed in the study - dawn and dusk.
It is already known that some drugs work better at certain times of the day.
Heart disease is driven by artery-clogging cholesterol, which is mostly made in the liver at night. Taking statins in the evening makes them more effective.
The researchers said 56 of the top 100 selling drugs and nearly half of the World Health Organization's list of essential medicines acted on genes which were now known to have this daily oscillation.
Dr John Hogenesch told the BBC News website: "I'm hopeful that we can use this information to design better therapies with existing drugs, and that's huge because it's not going to cost any more money.
"I think there is a real opportunity to improve current medication in a way that will be impactful."
Dr Simon Archer, a body clock scientist from the University of Surrey, told the BBC: "If you move away from one tissue, we looked at gene expression just in the blood, and look at the whole organism then that precise temporal organisation applies to much more than people previously realised.
"If 40-50% of genes are going up and down over 24-hours and these are drug targets, then it's going to be important.
"Thousands, millions of people potentially, could benefit from taking their medication at a different time of day and raising this kind of awareness is important."
Prof Andrew Loudon, from the University of Manchester, said it was a "really important" study.
He said all drugs were a trade-off of benefits and side-effects, such as liver damage.
"Drug companies do everything they can to make long acting pills that can be taken once a day, but we need targeted drugs with a short half-life and time them for maximal effect and minimal side-effects."