Two major explanations for why we sleep are restoration and memory consolidation.
Restoration: During sleep the body repairs tissues, supports the immune system, and restores resources needed for the next day.
Memory consolidation: Sleep stabilizes and strengthens newly formed memories, which is one reason sleep is an important part of studying.
Activation-synthesis theory says dreams are the brain's attempt to make sense of spontaneous neural activity during sleep, especially REM, so bizarre stories need no hidden meaning.
Consolidation theory says dreams are connected to memory consolidation, so pieces of waking life like people, places, and worries show up in them.
Key terms
restoration
Explanation for sleep: the body carries out maintenance and repair and restores resources while we sleep.
memory consolidation
Stabilizing and strengthening newly formed memories so they're more likely to last.
activation-synthesis theory
Dreams result from the brain organizing random neural activity during sleep into a meaningful story.
consolidation theory
Dreaming is connected to memory consolidation, as the brain processes and organizes information from our experiences.
Watch out
The key difference is where dreams come from: activation-synthesis = random brain activity; consolidation theory = processing our experiences.
Don't mix up why we sleep (restoration, memory consolidation) with why we dream (activation-synthesis, consolidation theory).