The paper aimed to show effect of period, when cows were kept in pens with a free-stall system, on lying time and other form of behaviour, including one circumstance: bedding material (sand) was not delivered during the investigation period. Two technological groups of dairy cows kept in the barn with a free-stall system were investigated. Each group of cows showed decreased lying time in the analysed period. The paper proposed a method of approach to analyse the research data. The results of analysis indicate that relationship between combined periods (doubled days) and lying time can be presented with a linear function with higher value of determination coefficient in comparison with single days. Doubling days in the constant measurement of lying time of cows translated into the possibility of describing a dependent variable (lying time of cows in the considered period) with a linear model which differs a considerably high value of the coefficient of determination in comparison to the option, which includes single days.