April 02, 2016

Why do we forget our dreams?

There's no time to lose, I heard her say Catch your dreams before they slip away - The Stones (Ruby Tuesday)

Everyone dreams while sleeping. However, most people struggle to recall their dreams. Even if a person remembers a dream, it is generally just a fragment of it. How much and how often can one remember varies significantly. There are also considerable individual differences in abilities to recall dreams. So, why is it that people tend to forget dreams?

It is clear that since it is a normal phenomenon, the brain is wired to forget dreams. Some researchers say that it may be to prevent information overload. But, to make things even more complicated, science still has very little understanding of the function of dreams.

Dreams and sleep cycles

There are four stages of sleep. The first three stages are called non-REM stages (non-rapid-eye movement). In the first stage, a person falls asleep, and the second non-REM stage is light sleep. And finally, the third non-REM stage of deep sleep occurs, which has a highly refreshing effect on humans.

These three non-REM stages are followed by the so-called REM (rapid eye movement) stage of sleep. It is a stage when person dreams. That is why there is rapid eye movement. This stage starts after about 90 minutes of non-REM sleep. Initially, it may last for about 10 minutes only. However, during the night, several cycles of non-REM and REM are repeated, and each time REM cycle starts getting longer.

It means that the more the person sleeps, the longer the REM cycle and the longer a person's dream. During dreams, a person may go through various vivid experiences. And yet, most of these experiences are forgotten on getting awake. So, what is happening inside the brain?

Why do we forget dreams?

It appears that, slowly, science is starting to understand why we forget dreams and the underlying mechanism. They think that the brain works actively to erase dreams by sending some specific signals. It could be a way to prevent information overload, delete unnecessary information.

Early studies have shown that part of the brain called the hippocampus plays a vital role in staying active, appetite, learning and memory. Hippocampus particularly plays a central role in information storage.

They know that one of the severe sleep disorders called narcolepsy occurs when some of the neurons in the hippocampus fail to secrete a hormone called orexin/hypocretin. These hormones are essential for staying awake. And in their absence, a person may fall asleep involuntarily, like in narcolepsy.

Thus, researchers focused on the hippocampus, understanding that it plays a critical role in sleep and memory. In one of the studies, they found that parts of the hippocampus become highly active and start secreting melanin-concentrating hormone (MCH) during REM sleep. MCH has a vital role in sleep and appetite.

But now researchers think that high activity of MCH and its increased production in the hippocampus during REM sleep may help explain why people forget dreams. This hormone appears to prevent the consolidation of information, its transformation into memories. It means that during REM sleep brain actively tries to forget dreams and eradicate them.

To prove that MCH is a hormone responsible for eradicating memories, they suppressed its production in the hippocampus in mice. They found that suppressing MCH boosts memory. It means that increased output of MCH during REM indicates that the brain actively tries to erase dreams to prevent information overload.

It appears that there are also other mechanisms involved that may explain why a person can only partially remember dreams. Acetylcholine (ACH) and norepinephrine (NE) are two of the most vital neurotransmitters. Acetylcholine is essential for maintaining brain plasticity, whereas norepinephrine plays an important role in memory formation.

When a person is awake, the brain stem continuously sends signals to higher brain centers with the help of ACH and NE. Thus, actively maintaining the brain's activity and promoting the learning process.

New studies show that levels of ACH and NE start declining when the brain is resting. Reducing their level in ascending pathways help reduce the activity of higher brain centers, reducing learning ability. It is logical, as the brain does not need to learn much while asleep.

Studies show that the levels of both ACH and NE start falling in ascending pathways during non-REM sleep. Their level is lowest in the third stage of non-REM sleep, that is, during deep sleep. It is also the time when the brain is at its calmest levels and has a very low level of activity.

However, researchers noted that during REM sleep (that is while dreaming), the activity of ACH rises again, but not activity NE. Thus, ACH helps boost brain activity, its plasticity. However, a low level of NE prevents the consolidation of information in the brain.

However, it is worth understanding that science has still to understand why people forget dreams. For example, most people may especially remember early morning dreams. It could be because, during later sleep cycles, the REM stage lasts longer and thus the dreams. Further, there is increased activity in some brain centers responsible for consciousness.

One of the most complex and paradoxical phenomena is that of lucid dreams. It is a condition when a person is not only aware of dreams but can also control activity while dreaming and still remain asleep. Nonetheless, despite much higher control and consciousness level during lucid dreams, one still struggles to remember them vividly. It clearly shows that the brain is actively trying to get rid of the extra information.

To conclude, though people can remember fragments of dreams, most dreams are forgotten on getting awake. Thus, it appears that the human brain is actively erasing information to prevent information overload.