Why do we sometimes remember what we dream and sometimes not?

It's a very common question. The truth is that we all dream every night, even if we don't always remember it.

This is because sleep is not a single state; it is composed of several phases that alternate continuously. One of these phases is REM (Rapid Eye Movement) sleep, which occurs cyclically during the night, approximately every 90 minutes. REM sleep is associated with most of our dreams and becomes more frequent as the night progresses. Dreaming is related to memory, which is facilitated during the REM phase, albeit transiently, which may make dream recall less durable.

Understanding this helps us understand why we sometimes don't remember our dreams. If we do not wake up during REM sleep, we are less likely to remember what we dreamt. Also, the emotional intensity of dreams, as in the case of vivid dreams, facilitates their recall. Paradoxically, some factors that disturb REM sleep may, in certain circumstances, decrease the ability to dream or remember dreams, and in others, increase it.

For example, women, especially young women, introverted people, hormonal changes such as early pregnancy and sleep fragmentation can influence this phenomenon. Lack of sleep due to sleeping too few hours can reduce the opportunity to go through all phases of sleep and thus remember what we dream. 

In addition, mood disorders such as anxiety and depression, mental disorders such as schizophrenia, and sleep disorders such as apnea (snoring with pauses in breathing), insomnia and abnormal movements during sleep can also affect our sleep cycle.Other causes can be health problems such as narcolepsy and the use of certain medications or recreational drugs, such as alcohol.

So when does it become a health problem to not dream or to be disturbed by vivid dreams? The answer depends on the level of nocturnal disturbance and the presence of problems that affect our sleep, such as inadequate sleep schedules, sleeping fewer hours than necessary, use of stimulants, sedentary lifestyle, lack of a relaxing bedtime routine, and excessive stimulation with electronic devices. If these nocturnal problems affect our performance during the day, causing tiredness or difficulties in study or work, or if they are associated with severe snoring, it is advisable to consult a specialist.

That said, whether or not we remember dreams depends on the phase of sleep we are in when we wake up. Some factors and disorders such as those mentioned above can affect our sleep cycle and therefore vary the ideal percentage that each sleep stage should have. Thus, for example, our percentage of REM sleep can be diminished and thus, decrease the possibility of remembering our dreams.