Introduction:
This study hypothesized that the lockdown due to the COVID-19 pandemic implemented in Poland on 11 March 2020 negatively influenced the lifestyle, dietary and sleeping habits, and physical activity of Polish children. The study aimed to assess the change of children’s lifestyle habits, and screen and sleep time due to measures taken during this lockdown.

Material and methods:
The study was cross-sectional and involved 486 parents of children aged 3–18 years, who completed the questionnaire containing 37 questions related to sociodemographic factors, nutrition, physical activity, and screen and sleep time regarding the child before and during lockdown.

Results:
During the lockdown, the number of meals consumed daily increased significantly in children between 7 and 11 years old (0.19 ±0.68, p < 0.001) and 12 and 18 years old (0.24 ±0.81, p = 0.005), whereas it decreased in the youngest group (3–6 years old) (–0.11 ±0.66, p < 0.001). The declared consumption of fast food decreased in all groups (p < 0.001), whereas the number of responders who prepared meals at home increased significantly (95.9% vs. 98.4%, p = 0.02). The time of physical activity decreased significantly during the lockdown in the groups of older children (p < 0.001), especially in children who live in blocks of flats. In the youngest group of children, the declared time of physical activity did not change significantly. In all groups of children, the screen time increased significantly during the pandemic. The amount of sleep also increased in all age groups but its quality worsened.

Conclusions:
It can be concluded that the lockdown due to the COVID-19 pandemic generated mainly undesirable lifestyle changes (decreased physical activity, increased screen time); however, desirable effects (increase in meals eaten at home and amount of sleep) were also observed.