South African sprinter Oscar Pistorius claims he mistook his girlfriend for an intruder after waking during the night. He shot her to death through a bathroom door. It wasn’t until after the lethal shots were fired, says Pistorius, that he realized Reeva Steenkamp was not in bed. It will be up to the court to determine whether Pistorius’ story is true. But in general, why are people confused when they first wake up?
Because your brain wakes up in stages. Psychologists describe the disorientation in the period immediately after waking as “sleep inertia.” Functional imaging shows that, when you are roused from sleep, different parts of the brain spring into action at different rates. The brain stem, which controls such basic functions as breathing and heartbeat, immediately ups its activity level. The thalamus, which processes sensory inputs, and the anterior cingulate cortex, which plays a role in basic awareness, are also quick to awaken. The prefrontal association cortices are among the latest risers. This portion of the brain is heavily involved in higher-level cognitive processing, like figuring out where you are.
Sleep inertia is most intense during the first five minutes after waking up. Studies show that freshly woken individuals have reduced reaction time, poor memory and grip strength, and are exceedingly bad at math problems. In one study, participants were asked to combat a computer-simulated fire. (Researchers chose this task because many emergency personnel make critical decisions moments after waking up.) Subjects who had woken up three minutes before being thrust into the virtual fire saved only about half as much land from the conflagration as fully rested people. The effects of sleep inertia in the study were also surprisingly sustained: Participants performed poorly even 30 minutes after waking up. Other studies have suggested that the effects of sleep inertia can be detected as much as two hours after waking.
There is a widespread belief that being jolted awake from deep sleep, also known as slow-wave sleep, produces a more intense and prolonged period of disorientation than waking during REM sleep. Popular smartphone alarm apps, which purport to pick the best moment to wake you, are based on this concept. The research on the topic, however, is mixed. Some studies support the connection between sleep stage and sleep inertia, while others find no connection.
It’s unlikely that Pistorius could use sleep inertia to escape a murder conviction. Successful sleep-based defenses typically involve more serious disorders than ordinary grogginess. A Canadian man who drove 14 miles to beat his mother-in-law to death with a tire iron in 1987 won acquittal by claiming he sleepwalked through the incident. Briton Jules Lowe was acquitted of murder charges eight years ago by claiming he was either asleep or in a state of “confusional arousal” when he beat his father to death. Confusional arousal is an extreme form of sleep inertia, analogous to the pediatric phenomenon of “sleep drunkenness,” when kids awake from deep sleep, stumble aimlessly around their home, and sometimes talk gibberish. Confusional arousal is linked to heavy drinking, and reports suggest that police found evidence of alcohol consumption in Pistorius’ home on the night of the killing. This defense probably wouldn’t work for the sprinter, though. People who suffer from confusional arousal rarely remember their actions, while Pistorius’ affidavit demonstrates a vivid memory of the killing.
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