In the latest issue of Nature I found a review  on the book entitled "Why Life Speeds Up As You Get Older" . "Life speeds up." -- This seems to be an appropriate expression. The appropriateness comes quite naturally; the author of the book, Douwe Draaisma, is a historian of psychology at the University of Groningen in the Netherlands. I wanted to know what was the author's answer to the question "Why does life speed up as you get older?" and read the review.
The reviewer, Yadin Dudai of the Department of Neurobiology, Weizmann Institute of Science in Israel, writes a brief introduction to the science of memory in the first half of his review. Note that the subtitle of the book reviewed is "How Memory Shapes Our Past." So, I noticed that the central theme of the book is not human perception of time but memory.
From the review we learn the followings: Memory can be classified into "declarative memory" and "non-declarative memory." The latter refers to bodily memory such as habits and modified reflexes. The former refers to conscious memory, and includes "episodic memory" and "semantic memory." "Episodic memory" is mental travel to the personal past, usually involving some re-experienced emotion. "Semantic memory" is acquired information transparent to conscious awareness and not always related to unique personal experience.
Dudai then tells us these: Draaisma's book reminds us that an interest in memory is primarily synonymous with a wish to understand the joy and sorrow of personal memory, i.e., "episodic memory." The book is a fine collection for memory lovers who will appreciate the facts it contains as well as rich metaphors. The title of the book comes from one of touching essays included in it. -- Oh, I could not learn the answer to the question "Why does life speed up as you get older?" --
Then I sought other reviews on Draaisma's book at Amazon.com Web site. There was just one good review written by a customer. The writer, Rob Hardy, is a psychiatrist and is ranked at the 50th among Amazon.com customers who send reviews (by the way, I am the 3537th). He kindly writes what I wanted to know as follows: There is not a fully accepted reason for that question. William James explained in 1890 that in youth, there were novel experiences, something new every day, but that every passing year brought routine that smoothed the days, weeks, and years into a collapse of time. A period full of memories, viewed in retrospect, seems to expand and be fuller and longer. [William James (1842 - 1912) was an American philosopher and psychologist, and was the novelist Henry James's brother.]
- Y. Dudai, Nature Vol. 434, p. 823 (2005).
- D. Draaisma, Why Life speeds Up As You Get Older: How Memory Shapes Our Past translated by A. & E. Pomerans (Cambridge University Press, 2004).
- R. Hardy, Evaluation of Our Real Memories (Amazon.com Web site, 2005).