Most dreaming happens during REM sleep, which makes up 20–25% of the night; if vivid dreams keep leaving you exhausted, scientists say the real culprit may be something else, keeping you from restorative sleep

Not everyone who closes their eyes and hits the bed at night goes to sleep. Some end up transitioning into a dream-like state that has them waking up while feeling hazy, lazy and heavy, with no feelings of rest and freshness. But is it the dreams that weigh you out or does it have something else to do with you?

A dream that you remember?

If you wake up during or just after a REM period, you are more likely to remember what you were dreaming.

Most of the dreams one remembers are the ones they see with their eyes open. When asleep, most dreaming occurs during rapid eye movement (REM) sleep, which makes up 20-25% of our total sleep time. Humans have four to six rounds of such REM throughout the night, with each round growing longer as the morning approaches.Everyone dreams and most of us dream multiple times a night, whether we remember it or not. If you wake up during or just after a REM period, you are more likely to remember what you were dreaming. Whether you remember a dream or not can also depend on the emotional intensity of the dream and whether you briefly wake up during the night. It also depends on how every individual’s brain stores memories overnight. People who remember vivid, emotionally intense dreams tend to have lighter, more broken sleep.

What happens in the brain when you dream?

During REM sleep, your brain is running almost as hard as it does when you are awake, firing away while the rest of your body lies completely still. Your muscles are essentially in a state of paralysis, which stops you from acting out the events of your dream. At the same time, the parts of the brain that handle emotion, the amygdala, hippocampus and thalamus are extremely active. The prefrontal cortex, which normally keeps things rational and logical, is much less engaged. This is why you get vivid, emotionally charged experiences that feel completely real but make absolutely no logical sense.While most people believe that dreams are brief, fragmented flashes through the night, the truth is otherwise. Evidence suggests that REM sleep dreams appear to unfold roughly in real time. When researchers have woken people from REM sleep and asked them to describe their dream, the length of their account closely matches the duration spent in the dreaming stage of sleep (REM episode). A dream that feels like 20 minutes was probably about that long in real life.The place where people go wrong, is estimating how much of their night they spent dreaming. A stressful or vivid dream feels longer and stays with you even when you have woken up. On the other hand, a dull one vanishes before you even open your eyes.Moreover, we tend to remember dreams that we woke up during. Someone who was sure they dreamed all night probably had a completely normal night of REM sleep. They just happened to wake during the emotionally charged parts, and those are the ones that stuck.

Why do you wake up exhausted after dreaming?

If you remember a dream, you almost certainly woke up during it. Those wake-ups, even the ones you barely register, take time away from deep sleep.

During REM sleep, the brain is not resting the way deep sleep allows. Even so, brain imaging studies suggest this energy use alone is not responsible for the fatigue people feel after a night of heavy dreaming. Dreaming on its own does not seem to impact a person’s sleep quality unless it tips into nightmares.The simple explanation is this: if you remember a dream, you almost certainly woke up during it. Those wake-ups, even the ones you barely register, take time away from deep sleep.They also give the brain less opportunity to clear a waste product called adenosine. During the day, adenosine builds up in the brain. As it accumulates, the pressure to sleep furthers. One of sleep’s main jobs is to flush this out, and it does that most effectively during deep sleep. Wake up before the task is finished and you might find yourself more tired than usual.Waking from REM sleep is also harder on the body than waking from lighter stages. It can produce sleep inertia, that thick, foggy state in which your brain refuses to come online. The tiredness is not a consequence of dreaming: it’s a consequence of when you woke up and what stage you were pulled from.

A rebound at sleep

When sleep is repeatedly broken or cut short, the brain makes up for the lost REM time on the night coming further, spending a higher proportion on sleep in that stage. This is known as REM rebound, and it is a compensatory response rather than a problem in itself. The actual problem is whatever is causing the sleep disruption.If you regularly remember most of your dreams, feel like the number of dreams you have has increased, or find yourself waking up tired most mornings, your fragmented sleep may mean the brain isn’t getting the deep, restorative stages it needs.