Have you ever been told in the morning that you did something in the middle of the night before that you have absolutely no recollection for? Have you ever been positive you set your alarm but woken up after it was supposed to go off, not remembering having ever turned it off yourself? Instances like these are often due to the memory-eroding properties of sleep, and in particular a phenomenon known as retrograde amnesia.
Retrograde amnesia refers to the loss of memory for things preceding a certain event. When we talk about retrograde amnesia in the context of sleep, this event is sleep onset. Dr. D gives a more thorough explanation below:
Because of this apparent lack of memory consolidation, we often can't remember exactly the things we did or saw up to five or six minutes before we fell asleep. Even if you were to wake up only 10 minutes or so into your sleep, chances are you would experience retrograde amnesia and forget events that happened only 11 or 12 minutes prior.
The following graph reflects this probability, from a study where words were presented to participants at various minutes before sleep onset:
Two interesting points emerge from this graph. First, we can see that the longer before sleep onset the words were presented, the greater percentage of words were recognized after the awakening. So to put it in a more everyday scenario, I may remember what happened in the book I was reading five minutes before I fell asleep, but I may not remember the chain of thoughts it sparked after I put the book away and began to close my eyes. Sound familiar?
Interesting Stuff!What happens when you are reading the book virtually all the way up until you fall asleep, your face passing out on top of the pages? Have you ever gone back to that book the next day and not really remembered where you left off, re-reading areas that later start to feel vaguely familiar? That's retrograde amnesia at work.
Second, we also see from the graph that the subjects awakened only 30 seconds after sleep onset showed little to no memory impairment. Apparently it is not sleep onset itself that disrupts the memory, but that it takes a small bit of time for those memories to fade even after sleep has been initiated.
When you think about it, can you capture the moment of sleep onset in your mind? I mean, you experience it just about every night of your life. Do you know what it feels like, really remember what it feels like, to go from awake to asleep? It's a very difficult thing to do.
The passage from awake and aware to asleep and incognizant is instantaneous and virtually impossible to recall or examine retrospectively. Why is that? Probably partly due to the very fleeting nature of the moment itself, but it's also surely due in large part to retrograde amnesia.
It's rare that most of us think about the things that happen to us in the couple minutes leading up to sleep. It's usually pretty trivial stuff that goes on as we settle into sleep mode (if it was that exciting it would be harder to fall asleep!). The more common way most of us experience retrograde amnesia is in the middle of the night, when a brief awakening stems to some kind of activity that we can't remember in the morning.
Honey, why were you looking around for your golf clubs last night at 2 a.m.?
What on earth are you talking about Dear???
You don't remember? I definitely saw you, and there's no chance you were sleepwalking. You were perfectly awake.
Other common examples of retrograde amnesia affecting what events you can and can't remember in the morning, despite being completely awake when they happened, include specifics of middle of the night phone calls, things your bed partner told you when you both woke up momentarily, and the fact that you woke up momentarily at all.
Most all of us have experienced some type of instance where we had no clue something done in the middle of the night actually happened until someone who witnessed it insists to us that it did. With the concept of retrograde amnesia under our belts, we can now understand why that is.
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Dr. Dement's pioneering textbook has been the core text for Sleep and Dreams since 1980, but it has just recently been made available to the wider public for the first time.
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