Published in the journal Sleep, the researchers used objective sleep measures to show that people who sleep six hours a night or less are four times more likely to catch a cold, compared to those who sleep more than seven hours in a night.

Aric Prather, assistant professor of psychiatry at the University of California, San Francisco, and lead author of the study, said that the findings add to growing evidence emphasizing how important sleep is for health.

"It goes beyond feeling groggy or irritable," Prather said. "Not getting enough sleep affects your physical health."

The findings in the journal Sleep are based on a study of 164 volunteers who allowed themselves to be exposed to a cold virus by researchers from UC San Francisco and the University of Pittsburgh Medical Center, who were also tracking their sleep habits.

In 2009, Carnegie Mellon University's Sheldon Cohen found for the first time that insufficient sleep is associated with a greater likelihood of catching a cold. To do this, Cohen, who has spent years exploring psychological factors contributing to illness, assessed participants self-reported sleep duration and efficiency levels and then exposed them to a common cold virus.

Prather approached Cohen about the possibility of investigating sleep and susceptibility to colds using data collected in a recent study in which participants wore sensors to get objective, accurate sleep measures.

"We had worked with Dr. Prather before and were excited about the opportunity to have an expert in the effects of sleep on health take the lead in addressing this important question," Cohen said.

In the new study, the volunteers first underwent health screenings and gave detail of their lifestyle so researchers could understand factors such as stress, temperament, and alcohol and cigarette use.

Their sleep habits were measured for one week before the beginning of the study. Once sequestered in a hotel, researchers administered the cold virus via nasal drops and monitored the volunteers for a week, collecting daily mucus samples to see if the virus had taken hold.

The results showed those who had slept less than six hours a night during the week before the study were 4.2 times more likely to catch the cold compared with those who had more than seven hours of sleep.

Those who slept less than five hours were 4.5 times more likely to get sick.

"Short sleep was more important than any other factor in predicting subjects' likelihood of catching cold," said Prather.

"It didn't matter how old people were, their stress levels, their race, education or income. It didn't matter if they were a smoker. With all those things taken into account, statistically sleep still carried the day."

Prather said the study shows the risks of chronic sleep loss better than typical experiments in which researchers artificially deprive subjects of sleep, because it is based on subjects' normal sleep behavior.

"This could be a typical week for someone during cold season," he said.

The study adds another piece of evidence that sleep should be treated as a crucial pillar of public health, along with diet and exercise, the researchers said. But it's still a challenge to convince people to get more sleep.

"In our busy culture, there's still a fair amount of pride about not having to sleep and getting a lot of work done," Prather said. "We need more studies like this to begin to drive home that sleep is a critical piece to our well-being."

Previous studies have linked lack of sleep to chronic illness, premature death, susceptibility to disease, car crashes, industrial disasters and medical errors.

"Overall, we expect global growth to remain moderate and likely weaker than we anticipated last July. This reflects two forces: a weaker than expected recovery in advanced economies, and a further slowdown in emerging economies, especially in Latin America," IMF chief Christine Lagarde said in a speech at the University of Indonesia.

"Asia as a region is still expected to lead global growth. But even here, the pace is turning out slower than expected with the risk that it may slow even further given the recent spike in global risk aversion and financial market volatility," she said.

The IMF cut its global economic forecasts in July to a growth of 3.3 percent in 2015, down from 3.4 percent in 2014. The projection compares with a 3.5 percent forecast made by the IMF back in April this year and 3.4 percent in July 2014. The next global health report is expected in October.

Besides the slowdown in pace of growth in developing economies, Largarde said there are other contributing factors for the lower growth projection including lower commodity prices which is going to adversely impact several developing economies.

"Commodity prices have come off their peak, and this decline is projected to persist," said the IMF chief, who is visiting Indonesia for the first time in three years. She warned that the trend is likely to see weaker demand for goods from emerging economies like Indonesia that "for some time to come".

"There is yet a third shiftunfolding in advanced economies. There are signs that the recovery is firming up in the United States, advancing the prospects of interest rate lift off. This could pose a risk for emerging economies, including Indonesia, in the form of weaker capital flows, higher interest rates, and financial volatility," Lagarde stated.

The changed outlook by IMF follows a sharp sell-off in China's stock market amid waning confidence about the country's economic outlook.

The IMF managing director sought to play down fears that China's authorities are out of policy moves to shore up their slowing economy while warning emerging economies like Indonesia to "be vigilant for spillovers" from China's slowdown.

"As the Chinese economy is adjusting to a new growth model, growth is slowingbut not sharply, and not unexpectedly. The transition to a more market-based economy and the unwinding of risks built up in recent years is complex and could well be somewhat bumpy. That said, the authorities have the policy tools and financial buffers to manage this transition," Lagarde stated.

Fears of a slowing Chinese economy impacting neighbouring countries have led to a sharp weakening of their currencies. Lagarde sought to reassure Indonesia, Southeast Asia's largest economy, that it had the "right tools to actually react" to the global volatility.

"You have very sound public finances with overall government debt in the range of twentyish percent relative to GDP, you have a relatively small deficit," she said, ahead of her meeting with Indonesian President Joko Widodo.