In a statement, Jammeh said the shorter work week, with Fridays off, will give the country's mostly Muslim population more time to pray, socialize and tend to farming. The change to four 10-hour work days was effective Friday.
Critics say the decision will promote laziness and disrupt Gambia's economy, based on peanut exports and tourism, specifically its beaches.
Thursdays and Fridays, or Fridays and Saturdays, are typically non-working days in the Muslim world, the BBC said, conflicting with Saturdays and Sundays off in the western world.
The BBC noted Jammeh, who took power in Gambia in a 1994 bloodless coup, is known for eccentric behavior.