Your mental clock is exceedingly accurate. Unless you have been knocked unconscious, hypnotized, or otherwise interfered with, you always know what time it is, down to the second. You can also measure any elapsed time with the same accuracy. Sleep does not interfere with this, and you can wake up at a predetermined time if you choose. Changes in time zone have no effect, nor does interplanetary travel, although you may need some time to adjust to the new "day."