On July 4, 2012, approximately 0340 eastern daylight time, a Robinson R44 helicopter, N561CH, was substantially damaged when it impacted a lake while maneuvering near Tallahassee, Florida. The certificated private pilot was not injured. Night visual meteorological conditions prevailed, and no flight plan was filed for the flight, which departed Tallahassee Regional Airport (TLH), Tallahassee, Florida, about 0330. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. The pilot stated that he had woken up at 0900 the morning of July 3, and he and another pilot had driven approximately six hours to West Palm Beach, Florida, to pick up a helicopter and fly it back to TLH. The flight from West Palm Beach to TLH was conducted from about 2230 – 0200. Upon arriving at TLH, the pilot decided to take the accident helicopter on a short flight to build night flight time and conduct a landing in order to meet night proficiency requirements. After departing TLH, he followed a road to the northeast at an altitude between 600-800 feet mean sea level for about 8 minutes, before turning back towards TLH. The turn was conducted over the pilot’s residence, which was located next to a lake. During the turn, the pilot saw the clutch actuator light illuminate and remain illuminated for 9 seconds. The pilot reached for the circuit breaker box under the passenger seat to pull the clutch circuit breaker, and then felt "light in the seat." He stated that the helicopter was rapidly descending, and he increased collective pitch to arrest the descent. After increasing collective pitch, he heard the low rotor rpm horn sound and then observed the surface of the lake reflecting the moonlight about 50 feet below the helicopter, as it continued to descend. The pilot increased collective pitch to soften the impact and the helicopter came to rest in the lake. The pilot then egressed the helicopter and swam to shore. The helicopter was recovered from the lake on July 7, 2012 and moved to a secure location for further examination. According to FAA records, the helicopter was manufactured in 2008, and was equipped with a Lycoming O-540, 260 hp, reciprocating engine. According to maintenance records, its most recent 100-hour inspection was completed on May 29, 2012 at a total time of 1298.6 hours. At the time of the accident, the helicopter had accumulated 1,336 total hours. The pilot held a private pilot certificate with a rating for rotorcraft-helicopter, which was issued in March, 2012. He reported a total flight time of 204 hours, of which 181 hours were in the accident helicopter make and model, and 11 hours were at night. His most recent FAA second-class medical certificate was issued in April, 2012. The helicopter was examined on August 17, 2012. Control continuity was established from the cyclic and collective controls in the cockpit to the main rotor. Flight control and drive train continuity were confirmed from the anti-torque pedals to tail rotor gearbox through a fracture of the tail boom. The four v-belts were found in place on the sheave and were observed to be taut and in good condition. The v-belts and clutch actuator were removed and sent to the manufacturer, where they were examined on August 23, 2012. Visual inspection revealed no damage or unusual wear on any of the four v-belts. The clutch actuator was observed to be extended approximately one inch. After freeing the motor to rotate using hand pressure, the actuator was installed on a production test fixture, and operated normally in both directions. The down-limit switch functioned correctly, and subsequently shut off the motor when the down (belt-loosening) limit was reached. In the up, or belt-tensioning, direction, both spring switches activated simultaneously as designed. The actuator was cycled an additional two times and operated with no anomalies. The 0353 weather observation at TLH included winds from 190 degrees at 3 knots, 10 miles visibility, clear skies, temperature 23 degrees C, dew point 21 degrees C, and an altimeter setting of 30.00 inches of mercury. According to U.S. Naval Observatory Astronomical data for the morning of the accident, the moon rose at 2042 on July 3, and set at 0735 the morning of July 4. The moon's phase was a waning gibbous with 99% of its visible disk illuminated. The Robinson R44 Pilot’s Operating Handbook stated regarding the clutch actuator light: “CLUTCH – indicates that clutch actuator is on, either engaging or disengaging the clutch. When the switch is in the ENGAGE position, the light stays on until the belts are properly tensioned…NOTE: The clutch light may come on momentarily during run-up or during flight to retension the belts as they warm-up and stretch slightly. This is normal. If, however, the light flickers or comes on in flight and does not go out within 7 to 8 seconds, pull the CLUTCH circuit breaker, reduce power, and land immediately. Be prepared to enter autorotation. Inspect drive system for a possible malfunction.” According to FAA Advisory Circular AC120-100, “Basics of Aviation Fatigue,” a window of circadian low normally occurs between 3 a.m. and 5 a.m. This is a period of low alertness and performance and elevated operational risk. AC120-100 further states that, in many work environments, a large increase in error rates and accident likelihood occurs in the early morning hours between 2 a.m. and 4 a.m. that roughly coincides with the minimum of the circadian rhythm of core body temperature. The helicopter was recovered from the lake on July 7, 2012 and moved to a secure location pending further examination.