24 minshorter nights for Swedish babies in May than in January. Night sleep follows the light.
Nights in Sweden follow the light: the stretch from bedtime to morning wake-up is longest in January (about 11.12 hours) and shortest in May (about 10.73 hours), mostly because morning wake-up comes about 25 minutes earlier while bedtime barely moves.
Complete days rebuilt from parents' own logs (a day counts when a parent logged the wake-up, every nap and bedtime), babies aged 3 to 18 months with country inferred as Sweden from where the app is used; no settled-routine filter. The night is the time from the previous evening's bedtime to the morning wake-up (local time). Averaged within baby per calendar month, then across babies within each month of age, then across ages 3 to 18 with equal weight, so the monthly mix of ages does not drive the result. Shortest: May, 10.728 h; longest: January, 11.124 h (23.8 minutes apart). Two full years of days, October 2024 to September 2026, so every month appears twice; 43,705 Swedish babies (smallest month-by-age cell 491 baby-months).