Ever feel like it is 7am, even though the clock says 9am?
A team of researchers at a US university has designed a blood test that can measure a person's inner body clock within 1.5 hours, an advance that may help personalise medical treatments in the future.
The study at Northwestern University was published in the Proceedings of the National Academy of Sciences (PNAS), a peer-reviewed US journal.
The "circadian rhythm" governs all cells in the body, and is a burgeoning field of research.
Three US geneticists won the Nobel Prize for Medicine last year for discovering the molecules that drive the process.
This biological clock regulates "all sorts of biological processes, when you feel sleepy, when you feel hungry, when your immune system is active, when your blood pressure is high, when your body temp changes," said lead author Rosemary Braun, assistant professor of biostatistics at Northwestern University.
When the clock is not regulated properly, research has shown a link to diseases like Alzheimer's, heart problems and diabetes.
Other research has pointed to the possibility some medical interventions like chemotherapy or blood pressure drugs might be more effective if taken at a certain time.
For the current study, researchers took more than 1,100 blood samples from 73 people.
Samples were taken about every two hours, and gene activity was tested at each interval to see how it changed over the course of a day.
The research allowed scientists to decipher if a person's body clock was off, for example, by up to two hours.
All the data from the 73 people studied was computerised, and it revealed a pattern.
"What the algorithm told us, is that there were a small set of about 40 markers that could predict the time of day with great accuracy," said Ms Braun.
Using this algorithm, scientists only need to take two blood draws to have enough information to decipher person's body clock.
More research is needed before the test can be made widely available.
It opens a "whole range of possibilities in terms of investigating how precisely the circadian clock is related to all sorts of health outcomes," she said.