In a vivid dream you feel the ground, hear the voices, smell the air — and it all makes sense at the time. Only after waking does it look absurd. This is not a failure of the brain; it is exactly how the dreaming brain is designed to work.
The brain treats the dream as reality
During REM sleep the brain is almost as active as when you are awake. The crucial difference is that the regions that normally test "is this really happening?" are largely quiet. With the reality-check offline, the brain simply believes what it is experiencing. That is why nothing seems strange in the moment.
Why senses feel so complete
Dreams draw on the same sensory areas the waking brain uses. When those areas fire during sleep, you genuinely "experience" the scene with full sound, sight and touch. Adding to the vividness is the amygdala, the emotion center, which is highly active in REM — so dreams come with strong feeling, which deepens how real they feel.
Why logic drops out
The prefrontal cortex — the part responsible for planning, self-awareness and critical thinking — is comparatively inactive in most dreaming. Without it, the dreamer accepts impossible transitions and strange characters without blinking. This is also why time, place and identity are so fluid: the brain is improvising a coherent-feeling world, not logging a documentary.
Why some people wake up while dreaming: lucid dreams
Occasionally a sleeper becomes aware that "this is a dream" while still in it — a lucid dream. This happens when the prefrontal cortex partially switches back on. Lucidity is learnable with practice (reality checks and dream journaling), and it is a well-documented, real phenomenon.