THE ACFT WAS ON A VFR X-COUNTRY FLT AT NGT, WHEN AN IN-FLT BREAK-UP OCCURRED. SUBSEQUENTLY, WRECKAGE WAS SCATTERED OVER ABOUT A 1.4 MI AREA. THERE WAS EVIDENCE THE LEFT WING FAILED IN POSITIVE OVERLOAD APRX 3' OUTBOARD OF THE WING ROOT. THE CABIN DOOR & TAIL SURFACES ALSO SEPD. ALL FRACTURES EXHIBITED EVIDENCE OF OVERLAOD. NO EVIDENCE OF ANY PRE-EXISTING DEFECT WAS FND. THE NON-INST RATED PLT WAS NOT IN CONTACT WITH ANY ATC FACILITY DURING THE FLT. RADAR DATA SHOWED THE ACFT MADE SVRL ALT EXCURSIONS DURING ABOUT A 30 MIN PERIOD BEFORE THE ACDNT. JUST BEFORE DISAPPEARING FM RADAR, THE FLT TRACK DEVIATED TO THE LEFT. THE ACDNT OCCURRED ON A DARK NGT AT ABT 2147 EDT; THOUGH HIS LAST REST PERIOD WAS NOT VERIFIED, THE PLT ROUTINELY SLEPT FM 2100 TO 0400. ABT 5 MI SW AT THE TRI-CITY RGNL ARPT, THE 2152 WX WAS IN PART: 5000 FT SCT, 8500' BKN, 30,000' OVC, VIS 7 MI, WND FM 130 DEG AT 7 KTS. ELEV OF THE WX STATION WAS 1519'. THE ACFT WAS AT 8700' MSL WHEN RADAR CTC WAS LOST.