What is the nature of dreams in A Midsummer Night's Dream?

Based on the play A Midsummer Night's Dream, Shakespeare might simply categorize the nature of dreams as enchanted, magical, just slightly beyond rational comprehension. Also from a thematic viewpoint in this play, Shakespeare presents a sort of message of "everything will be better in the morning" after a night of sleep and dreams.

Throughout the night, the spirits are playing tricks on and intervening in various ways in the interactions of the lovers in the woods. Puck, especially, uses his magic for comic effect to confuse and entangle Hermia, Helena, Lysander, and Demetrius, who chase each other and quarrel in various ways throughout the night. In the meantime, the world of the magical creatures and the mortals intersect  when Oberon commands Puck to cause the fairy queen to fall in love with the buffoon Bottom, who's transformed with the head of a donkey. After this night of confusion, mayhem, and magic, each of the lovers awakens back into their appropriate worlds with (arguably) their appropriate partners. Though confused about what exactly took place during the night, the characters are pleased with the happy ending and the clarity they all have now upon being awake.

When Bottom tries to explain what he's been through, he says:

I have had a most rare vision. I have

had a dream—past the wit of man to say what dream

it was. Man is but an ass if he go about to expound

this dream (4.1.190-193).

Bottom suggests that it's pointless to try to explain or understand the nature of dreams, but they fuel his creative mind, and he ties the plot from his vision into a new play.

Later, after King Theseus and his soon-to-be Queen Hippolyta hear the seemingly crazy stories of the lovers from their night in the woods, Theseus draws a comparison between madmen, poets, and lovers.

"The lunatic, the lover, and the poet

Are of imagination all compact" (5.1.7-8)

"Such tricks hath strong imagination,

"That if it would but apprehend some joy,

It comprehends some bringer of that joy" (5.1.19-21).

So, though Theseus himself is not convinced that dreams are magical, he notes that poets, lovers, and lunatics all claim that their creativity, love, or madness, in turn, is caused by external forces. Hippolyta is a bit more curious than Theseus, though, since she mentions how each of the characters have told similar and overlapping accounts.

While it's still debatable if there are actual magical or mystical forces at work, Shakespeare suggests that dreams are playful, mischievous, and not quite able to be understood, though a night's sleep and the promise of morning also bring about resolution to the previous night's troubles.