The implementation of sleep/usleep in Guile-1.8.7 uses select(), but that doesn't work on Windows using mingw:

guile> (sleep 1)
1                    ; returned immediately
guile> (sleep 4)
4                    ; returned immediately

On unix and on Windows under cygwin, these calls actually sleep, and return 0 normally.

Note: The select implementation on unix seems to rely on the "sleep pipe".  I submitted a bug report including a patch that removes that pipe on mingw (bug 27573), but sleep doesn't work even with the pipe enabled.

The fix is straightforward, making 'sleep' and 'usleep' call 'Sleep' rather than 'select'.  I assume that's how it was at some point in the past because scmsigs.c has definitions of 'sleep' and 'usleep' at the top of the file that are not currently used.

Suggested patch attached.