Train driver duties are diversified and irregular, as represented by the "two successive night duties," which involve a sign-off and a sign-on on the same calendar day. In the two successive night duties, train drivers are said to be subject to heavier strain than in other duty forms, as they are on duties successively during early morning hours and at midnight when their psycho-physiological function decreases. We investigated the influence of the length of home rest time between the sign-off and sign-on timeson the same day on their psycho-physiological function during the next on-duty hours. As a result, we thought that there was no influence on the next day, if moderate napping is inserted into the home rest time, and that it depends on the sign-off time of a duty whether drivers can nap enough at home sign-off time.