I would say if you meditate twice a day for 3 hour periods then i would say thats making up for any lack of sleep. Some people are actually unable to sleep at all and meditate in order to rest their body and mind however i would never recommend this instead of sleep. REM sleep in which we dream is invaluable towards body repair as it is the only time our brains neurochemicals become replenished during the mixed wave activity.
Its recommended for adults to sleep between 6-8 hours and this is considered healthy, the bare minimum its said we can get away with safely is 4 hours of sleep.
I wake up automatically after 6 hours of sleep. That's the way I've always been.