The study discusses changes in the sunshine duration in Poland, occurring in the years 1966–2018. The main analysis was carried out on a series of annual area sunshine duration, calculated from 11 stations, distributed relatively evenly throughout the area of Poland (variable UPLRK). A discontinuity was found in the course of UPLRK, consisting a quantum leap of this value in the years 1987–1989, and then the appearance of a statistically significant positive trend in the course of UPLRK. A change in the course of UPLRK and the total change in the sunshine duration regime occurred at the moment of change in ‘circulation epochs’, characterized by a change in the frequency structure of the mid-tropospheric circulation of macro-types W, E and C according to the Wangengejm-Girs classification. The frequency of these macro-types, by controlling the variability of the lower circulation (SLP fields), controls changes in sunshine duration. An increase in the frequency of the W macro-type, with which the UPLRK values are positively correlated, and a simultaneous decrease in the frequency of the E macro-type, with which sunshine duration is negatively correlated, which occurred at the time of change of macro-circulation epochs in 1987–1989, resulted in corresponding changes in the behavior of the sunshine duration process in Poland. Changes in the frequencies of the W and E macro-types are controlled by changes in the distribution of heat resources in particular waters of the North Atlantic. These changes are controlled by changes in the North Atlantic Thermohaline Circulation (NA THC). As a result, the changes in the UPLRK observed in the years 1966–2018 reconstruct changes in both the macro-circulation conditions in the Atlantic-European circulation sector and changes in the NA THC phases. This allows for a conclusion that the variability of UPLRK is a result of the internal dynamics of the climate system, and not, as it has been believed so far, the effects of anthropogenic changes in the concentration of aerosols in the atmosphere.