Researchers have found that the immune protein - tumour necrosis factor alpha (TNF-alpha) - is the reason why people feel sleepy while sick.
The study was conducted by a team of researchers led by Thomas Birchler at the University Hospital of Zurich, Switzerland.
The protein was known to set off inflammation in response to infection and some chronic diseases like rheumatoid arthritis and multiple sclerosis and was linked to fatigue.
Cancer patients, for example, treated with TNF-alpha sometimes reports severe lethargy while patients with sleep disorder, sleep apnoea sometimes report less daytime sleepiness after taking a drug that hinders TNF-alpha activity.
As part of the study, researchers gave TNF-alpha to mice and monitored the gene expression of genes participating in the biological clock.
The study found that during the day, the genes were expressed in their normal rhythm but the overall level of expression of some genes in mice on which the drug was administered was reduced.
"The oscillations were still in rhythm. But the output of the clock genes was much reduced," Nature quoted Birchler, as saying.
Researchers noted that the mice that received the protein looked tired and were less active and spent less time walking around their cages or on their running wheels.
According to immunologist Rodney Johnson of the University of Illinois, Urbana-Champaign the study has opened up an area that should be a fertile ground for research.
"We've known for years now that TNF-alpha induces changes in sleep patterns. This is the beginning of connecting it with genes involved in circadian rhythms," Johnson said.