Paul Pierce is opening up on his arrest ... and he's chalking the whole ordeal up to being "old" and "tired."
The NBA legend was taken into custody late Tuesday night ... after the California Highway Patrol said officers found him asleep at the wheel of a Range Rover SUV in the middle of U.S. Highway 101 in Los Angeles.
Pierce said in a statement on Threads on Thursday the CHP's decision to close lanes for nearly an hour to investigate a crash is what led to the whole issue.