... such stuff as dreams are made on ...

We sleep. Some less than others, and some so little they think they do not sleep at all. Sleep might be taken as a reward after the labours of the turn, or taken grudgingly if perceived as marcs wasted which could be used more fruitfully. And when we sleep, we dream. Again, some more than others. Some recall their dreams when they wake, others may not.
I wonder where they come from, these fantasies of the sleeping marcs? Maybe our mind sorts through memories of the turn just ended. Or perhaps it carries on considering ideas that had come to us, continuing to lay plans which we'd thought of in our waking marcs. It may be that there's a portion of our mind that never sleeps but sees all the turns and marcs of our lives, noticing all and forgetting nothing, putting memories aside in a place where our waking self may be unable to find them.
Whatever these dreams may be, wherever they come from, they can reflect our waking thoughts and act as the mirror of our life. We may relive the joy of pleasant times, recall visits with friends and acquaintances. Or they may grant the gift an extra opportunity to spend precious time with those who are dearest to us.
Others dreams, of course, are less pleasant. The terrors of the night, played out before our sleeping eyes as we relive darker events, times of loss and separation. Or perhaps they dwell on anxiety for the safety of loved ones.

And worst of all are those where we see the one we love in mortal peril - followed by the crushing realisation that this is no dream, and that peril in which we find our loved one is all too real.