Without obtaining a formal weather briefing or filing a flight plan, the certified flight instructor (CFI) and student pilot departed on a long night cross country instructional flight back to their home airport following a business meeting that was attended by the student pilot. Reported and forecast marginal visual flight rules (MVFR) conditions, and areas of instrument meteorological conditions (IMC), prevailed along their intended route of flight. This information would have been given to the CFI and/or student pilot if they would have requested a formal preflight weather briefing. Recorded radar data from the accident flight indicate that the airplane generally flew at an altitude that varied between 2,000 and 3,000 feet msl, which was consistent with an altitude that was intended to remain just below the reported scattered to broken layers of clouds along the route of flight. The flight path was along a nearly straight line to the destination airport. Due to radar coverage limitations, the radar data ended about 40 miles southeast of the accident site, when the airplane was flying about 2,200 feet msl, which was about 1,000 above the accident site elevation. A witness who lived in the vicinity of the crash site stated he heard the airplane fly over his home, with the engine sounding as if was operating at high power, followed by an impact sound. Examination of the crash site revealed that the airplane collided with trees and terrain in a steep nose-down attitude, and at a high velocity. The engine and forward portion of the airplane were embedded about four feet into the ground. Examination of the wreckage did not reveal any evidence of any preimpact mechanical malfunctions with the airframe, engine, or systems. Analysis of the wreckage and surrounding terrain revealed impact damage signatures consistent with a loss of aircraft control while under engine power. The direction of the impact was nearly opposite the intended route of flight. The flaps and landing gear were retracted, consistent with a cruise flight configuration. The accident site was located in remote, hilly terrain with very few lights. The accident occurred in dark night conditions, and in reduced visibility from a low cloud ceiling and possible localized fog. The airplane likely entered instrument meteorological conditions, or conditions in which a horizon was not visible, which led to a failure of the CFI and/or student pilot to maintain level flight. It is possible that the loss of control was precipitated by spatial disorientation; however, due to a lack of recorded radar data or flight recorder data at the time of the accident, the investigation could not determine whether the pilots became spatially disoriented. Additionally, the investigation could not determine who was manipulating the controls at the time of the loss of control; however, the CFI was the pilot-in-command of the airplane. Review of the CFI and student pilot's chronological sleep patterns and duty day revealed that both pilots were susceptible to the effects of fatigue based on their sleep lengths during the 24 hours prior to the accident, and based on their time awake before the accident. Research has shown that human fatigue can degrade performance and judgment, and this likely impaired the performance and judgment of the CFI and student. [ This report was modified on January 26, 2010 ]