The invention relates to a method for tamping sleepers of a track panel, supported in a ballast bed, by means of a tamping unit, comprising two tamping tools opposite one another which, during tamping of the respective sleeper, are lowered into the ballast bed with vibrations being applied, and are moved towards each other with a squeezing movement while the track panel is held in a raised position. In this, an evaluation device is used to monitor a squeezing speed of at least one tamping tool, with a current value of the squeezing speed being compared with a limit value when a predefined squeezing time or a predefined squeezing distance is reached, and with a notification signal indicating whether the current value is above the limit value. This may indicate that a void located below the sleeper has not yet been sufficiently filled.