Researchers say they have identified the chemical switch that controls the genetic mechanism regulating people's internal body clocks.
Although the process involves complex genes, the whole mechanism is controlled by a single amino acid - a building block of protein - they say.
It is hoped the discovery may lead to more effective drugs to treat sleep disorders and related ailments.
The University of California study appears in the journal Nature.
Lead researcher Professor Paolo Sassone-Corsi said: "Because the triggering action is so specific, it appears to be a perfect target for compounds that could regulate this activity.
"It is always amazing to see how molecular control is so precise in biology."
The body's internal clock, a highly sensitive mechanism able to anticipate changes in the environment, regulates a host of body functions, from sleep patterns to metabolism and behaviour.
It is estimated that it regulates up to 15% of all human genes.
Disruption of these rhythms can profoundly influence human health and has been linked to insomnia, depression, heart disease, cancer and neurodegenerative disorders.
The gene CLOCK and its partner BMAL1 control the body's internal clock.
The latest study found that a single amino acid in a protein produced by BMAL1 undergoes a modification that triggers the genetic chain of events involved with setting the body's rhythms.
The researchers found that if this modification is impaired in any way, the switching mechanism can be thrown off, undermining the whole system.
They are currently testing antibodies that can target the activity of this amino acid.
Dr Neil Stanley, a sleep expert at Norwich University Hospital, said 89 different types of sleep disorder had so far been classified, but all sleep medications - hypnotics - currently targeted the same neurotransmitter called GABA.
"People's sleep problems tend to be very individual, but we currently have a one-size-fits-all approach - our armamentarium is not exactly huge," he said.
"A new target may allow us to develop more specific treatments and to offer patients more personalised care."