An outside arbitrator has overturned the five-day suspension of a General Services Administration security guard who was found asleep at his post outside the Pentagon offices of the Joint Chiefs of Staff.

The embarrassing episode took place on a Sunday afternoon last summer. Two military security guards on routine patrol spotted the civilian guard asleep in a chair outside the top-secret offices of the nation's top military leaders. They alerted someone higher up the chain of command -- a Pentagon security officer -- who called GSA's Federal Protective Service, which supplies civilian guards.

According to the record of the case the Pentagon security officer then walked past the sleeping guard, inserted his pass in a special electronic pass-reading machine, got the green light and walked into the high-security area that contains many of the nation's top defense secrets. At that point, the record shows, the guard woke up and spotted the officer, but did not call him back to check his pass, considered a breach of security.

Military and civilian security brass were, to say the least, upset.

The security guard did not deny the allegations that he was asleep on duty. But he said that he had worked a heavy overtime schedule during the previous four days, and had also worked on his days off because GSA said it was short-handed.

He said he asked to be excused that day and was taking medicine for asthma, which he said can cause drowsiness. The guard, who had an outstanding record, said he was told to work but assured he would get a break during the shift. It was six hours, however, before relief arrived.

The guard admitted going to sleep at the end of the shift but said the punishment was too severe. The American Federation of Government Employees union defended him. An outside arbitrator was asked to make the final call.

Arbitrator Joseph M. Stone said that while there was no excuse for sleeping on duty, the penalty was too severe. He directed GSA to give the guard a written reprimand, but to restore pay and benefits lost because of the suspension.Pension Picking Session

Rep. Constance Morella (R-Md.) is sponsoring a town meeting at 10 a.m. tommorrow on the upcoming pension open season for federal workers. From July through the end of the year most employes must decide whether to remain in the current retirement program or move to the new Federal Employees Retirement System. The session will be in the Stella B. Warner County Office Building in Rockville. At 1 p.m. tomorrow on WNTR (1050 AM) Allison Sommerton of Chambers Associations will compare the old and new pension programs, and answer call-in questions from listeners.People

Michael G. Hansen has been named director of the Federal Executive Institute in Charlottesville. He succeeds Robert Matson in the Senior Executive Service job, which has a five-year term of office. Hansen is director of the key executive program at American University and has been a consultant to various federal agencies and the governments of Brazil and Argentina. The FEI is the government's top civilian training academy. About 1,000 fast-track U.S. executives go through the FEI each year.

Craig S. Brightup is the new director of the Federal Trade Commission's office of congressional relations. He had been political affairs manager for the U.S. Chamber of Commerce's southwest (Dallas) region.