Here’s what you’ll learn when you read this story:

• Highly superior autobiographical memory, or HSAM, is a remarkable ability that allows people to perfectly recollect every day of their life.
• In a new study, researchers conducted a polysomnography with nine HSAM individuals (and 13 others as a control) to try to understand what makes these individuals’ memories so superhuman.
• The results showed a higher density of sleep spindles, or bursts of brain activity, especially in the parietal cortex (a part of the brain associated with vivid recall).

Imagine if you could remember every moment of every day of your life with absolute clarity? Well, at least 100 people throughout the world don’t have to pretend. They live with highly superior autobiographical memory (HSAM), an incredibly rare condition that allows someone to recall every event of their life in perfect detail. Of course, there is a natural question that arises around HSAM, once you get over its slightly unnerving quality: Why do these people have such long-lasting and accurate memories while the other 8 billion of us slowly forget a majority of our life experiences day by day?

“Individuals with highly superior autobiographical memory (HSAM) have the rare ability to instantly recall nearly every day of their adult lives,” the authors of a new study investigating HSAM wrote. “Previous research indicates that the extraordinary abilities of HSAM individuals are not a result of better memory encoding. Rather, HSAM individuals appear to not forget as easily as other individuals.”

So, if it isn’t to do with encoding, it must have something to do with the brain’s memory consolidation, a process that occurs only when we sleep. Every night, slow-wave sleep—otherwise known as deep sleep or non-Rapid Eye Movement (NREM) sleep—presents the perfect opportunity for the brain to form long-term storage in the neocortex thanks to reduced external stimuli and an increase in neurotransmitters. A new study published in the journal Sleep looks more closely at NREM processes in an attempt to understand the recall gifts of HSAM individuals.

To do this, a team from Texas State University and Northwestern University conducted a polysomnography that included nine HSAM individuals and 13 control patients over the course of two nights. As part of the study, the researchers also administered a series of episodic memory, intelligence, and subjective sleep quality surveys. Interestingly, the HSAM group and control group showed no difference in these particular assessments, and there was also no difference in total sleep time, time spent in each sleep stage, or slow-oscillatory power in NREM (which is typically high amplitude but low frequency). However, once they looked at the sleep recordings themselves, differences began to emerge.

“HSAM individuals showed higher non-REM sleep spindle density than controls at frontal, central, and parietal sites, with the largest increase at parietal sites,” the authors wrote. “Furthermore, during non-REM sleep, spindles were more precisely coupled to the peaks of the slow oscillations in HSAM individuals than in controls at parietal sites.”

Sleep spindles—named for the marks they make on an electroencephalogram (EEG)—mark bursts of brain activity that occur exclusively during NREM sleep and are typically indicators of memory consolidation. The parietal cortex, which saw the highest increase of sleep spindles, is central to the Parietal Memory Network that supports vivid recollection. Interestingly, these spikes were also more accurately timed with slow oscillations, possibly making these bursts more efficient at transferring short-term memory to storage.

“These results are the first to demonstrate that aspects of sleep physiology previously implicated in memory consolidation,” the authors wrote. “This suggests that superior memory consolidation contributes to superior autobiographical memory.”

While remembering every day of your life might seem like a superpower, some memories are best forgotten. In a report by 60 Minutes in 2010, some individuals with HSAM call the ability “maddening.”

“I can’t just move forward,” Jill Price, who has HSAM, told 60 Minutes, “and that is detrimental to my daily life […]. [I]f I was like everyone else, my whole life would be very different.”

Whether memory is a gift or a curse, scientists are getting closer to understanding the mechanisms behind it.