SELF discusses two types of sleep conditions that fall under the “bad dream” umbrella: nightmares and sleep terrors.

There are few things that set nightmares and sleep terrors apart. For starters, you typically remember your nightmares—maybe not every detail, but you can remember that you’ve at least had one. But with night terrors, you could wake up and have no idea it occurred. The episode is typically remembered by your bed partner (hard for them to sleep through the screaming!), but not by you. So, if you sleep alone, you could have sleep terrors and not even know it.

A nightmare tends to get scarier gradually, not right away. Think of it like a scary movie—there tends to be a build up. Sleep terrors are different. You can be sleeping calmly, and all of a sudden you’re afraid and screaming.

This is partly due to the stage of sleep the event happens in. Nightmares occur during rapid eye movement (REM) sleep. This is the stage of sleep when vivid dreaming is most likely to happen, according to the American Sleep Association (ASA), and the brain is more active than it is during other stages of sleep.