Sleeping only a few hours at night may increase the risk for physical and mental strain when working in the heat, but a nap after lunch may help in some respects, suggests a small study from Japan.

Lead author Ken Tokizawa and his research team at the National Institute of Occupational Safety and Health in Kawasaki wanted to examine the effect of sleep loss in work situations such as construction sites.

“In the construction industry, falls are major safety problems,” said Tokizawa in an email to Reuters Health. He added that heat strokes may cause falls for workers.

Heat stroke itself is also a serious consideration, as it can result in damage to the heart, brain, muscle and vital organs, said Jonathan Moore, a lecturer in cardiovascular physiology at Bangor University in Wales, in email to Reuters Health.

Tokizawa and colleagues recruited 14 men who were in good health and had no sleep problems and studied them under four different conditions: after a normal night of sleep with no nap, after a normal night of sleep with a 30 minute nap after lunch, after sleeping four hours in a night with no afternoon nap, and after four hours of sleep, with the nap.

The lab test consisted of two 40-minute periods of walking in a 95-degree room, once in the morning and once in the afternoon. The conditions were meant to simulate the construction and manufacturing fields, so workers wore pants, jackets, boots and helmets during the experiment.

The study team assessed the level of physical strain by measuring participants’ rectal temperature, skin temperature and heart rate throughout the experiment.

The participants also rated their own levels of sleepiness, mental and physical fatigue, feelings of heat or cold, pleasantness and thirst.

Lastly, the researchers measured the subjects’ reaction times.

They reported their findings in Occupational and Environmental Medicine.

For the no-nap trials, people with restricted sleep had a higher core temperature during the afternoon walking period. People with less sleep also had higher skin temperature and sweating during the afternoon.

The people with limited sleep also reported more sleepiness and physical fatigue in the morning and afternoon and more unpleasant feelings in the afternoon. They also had slower reaction times after walking in the morning and in the beginning of the afternoon, though this leveled out as the afternoon session went on.

Taking a nap did not have an effect on core or skin temperature or sweating for people with restricted sleep, but people who took naps reported less sleepiness in the afternoon and felt less overheated and fatigued.

People who had taken a nap also displayed faster reaction times at the beginning of the afternoon session.

Moore noted in his email that napping seems to be beneficial for vigilance, but it may increase the risk of heat injury.

Napping did not lower core body temperature and “participants perceived themselves to be cooler than they really were during exercise-heat stress,” Moore said.

He feels that this finding requires further investigation.

Tokizawa also said that while taking a nap while running on limited sleep can recover some reduced alertness, it cannot protect against heat stroke.

“Be careful about heat stroke when you have lack of sleep, especially in the afternoon,” he advised.