A person's thinking ability may be better after being awake for 24 hours or being drunk than it is following a good night's sleep, a study suggests.
Some feel better upon waking than others
A University of Colorado team found understanding and short-term memory were worse in the minutes after waking.
Their finding has implications for workers such as doctors on night-duty, who are awoken and immediately asked to perform important tasks.
The study appears in the Journal of the American Medical Association.
After the study participants had had six nights of monitored sleep lasting eight hours per night, they were given a performance test that involved adding randomly generated, two-digit numbers.
Based on the results of this test, the researchers concluded the subjects exhibited the most severe impairments to their short-term memory, counting skills and cognitive abilities from sleep inertia within the first three minutes after awakening.
The most severe effects of sleep inertia generally dissipated within the first 10 minutes, although its effects were often detectable for up to two hours, they added.
None of the nine study participants had any medical, psychiatric or sleep disorders and were not using alcohol, nicotine, recreational drugs or caffeine.
They had also spent several hours each day practising the maths test used to quantify sleep inertia.
The study follows other research which has looked at the effects of going without sleep for over 24 hours - and found that has the same effect as being drunk.
Professor Kenneth Wright, who led the study, said: "This is the first time anyone has quantified the effects of sleep inertia.
"We found the cognitive skills of test subjects were worse upon awakening than after extended sleep deprivation.
"For a short period, at least, the effects of sleep inertia may be as bad as or worse than being legally drunk."
He said the explanation could be that certain areas of the brain take longer to "wake up".
Other research has found the prefrontal cortex - responsible for problem solving, emotion and complex thought - is one of those areas which takes longer to come "on-line" following sleep.
Professor Wright said doctors on night shifts, who may get awoken to treat an emergency, or ambulance workers and firefighters who may have to get up and drive to the scene of an incident, could be putting themselves - or others - at risk.
He said the study also illuminates the challenges faced by everyday people who are forced to make crucial decisions following abrupt awakening.
"If a person is awakened suddenly by a fire alarm, for example, motivation alone may be insufficient to overcome the effects of sleep inertia."
He said further research was needed to measure the effects of nap interruption and "recovery sleep" in on-call, sleep-deprived people.
'Waking time important'
Dr Neil Stanley, of the British Sleep Society, said: "There are a lot of people, like junior doctors, where it's not the number of hours they work that matters, it's when they sleep and how they feel when they wake up.
"Nobody should be doing anything really important for 15 to 30 minutes after they wake up."
He said people could perform less well than when they had been awake for a long time.
"If you are asleep, it's a much bigger transition to go from that to being awake than to stay awake, even for a long time, because then you'll be aware you are drowsy."