When we endure long periods of inactivity, the experienced time is heavy, slow, and boring. But because nothing novel or meaningful occurs, our brain records no new data. Therefore, the remembered time shrinks to almost nothing. We lived through a massive stretch of time, but retrospectively, it vanishes. Each individual moment of waiting or doing nothing may have felt like a drag, but the problem is that such larger extents of time are almost reduced to nothing when we introspect. The time was spent, but we have nothing to show for it. Conversely, when we are active and enjoying ourselves, the experienced time flies by. Yet, because we are laying down a dense track of new memories, the remembered time expands. Looking back, that brief, fast period feels rich, substantial, and long. Ultimately, our perception of how much time we have lived is at the mercy of our memory. If nothing happens, the time essentially did not exist. Thomas Mann discusses this phenomenon in depth in his book, The Magic Mountain. One of my favorite passages from the book: "Vacuity, monotony, have, indeed, the property of lingering out the moment and the hour and of making them tiresome. But they are capable of contracting and dissipating the larger, the very large time-units, to the point of reducing them to nothing at all. And conversely, a full and interesting content can put wings to the hour and the day; yet it will lend to the general passage of time a weightiness, a breadth and solidity which cause the eventful years to flow far more slowly than those poor, bare, empty ones over which the wind passes and they are gone. Thus what we call tedium is rather an abnormal shortening of the time consequent upon monotony. Great spaces of time passed in unbroken uniformity tend to shrink together in a way to make the heart stop beating for fear; when one day is like all the others, then they are all like one; complete uniformity would make the longest life seem short, and as though it had stolen away from us unawares."