unfortunately， we can’t customise the intensity of the flashing but only customise the led up and down.
maybe your can try a script to let the led down when the time at ninght by crontab like this :
edit /etc/crontabs/root
10 0 * * * echo 0 > /sys/class/leds/xxxx/brightness # turn off the xxx led in 0:10 everyday
30 8* * * echo 1 > /sys/class/leds/xxxx/brightness # turn on the xxx led in 0:10 everyday