Sleep is stored one row per stage, and the timeline read it like any
other type: the live path took the mean of a day's rows and the long
path the rolled-up month of them, so IN_BED, AWAKE and every CORE
fragment counted as a night, and a quarter of seven-hour nights read as
under two hours.
The sleep series now reconstructs nights the way the sleep page and the
day view do (CORE + DEEP + REM of the night's winning writer, keyed by
its wake day) and averages the bucket's nights, on every window length;
the rollups are not read for it. The stage rows are read a year at a
time with a day's margin on either side, so no read holds a decade of
rows and no night is cut where one read ends. A long window is cached
per record like the other long series.
