You are concentrating so intensely for hours. Most games started Saturday with a handshake. Then the timer starts. The timers actually have two clocks - one for each player. As one player finishes a turn, he flips a switch to turn off his clock and turn on his opponents.
Games run on a time limit - but can go as long as five hours.
Each player gets an initial 90 minutes for 40 moves. Time can be extended for an additional hour each. Many players measure their chess experience in decades. Yet, they sometimes are matched with players who themselves are not even 10 years old. And, often they lose to them.