ABHĀVA PRATYAYĀLAMBANĀ VRTTIR NIDRĀ.
That mental modification supported by cognition of nothingness is sleep.
So basically, you’re not thinking when you’re asleep, except for dreaming, and we’ll get to that a bit later. But sleep is still a vritti. It comes and goes and most of us can’t control it very well. Many of us sleep too much or too little, a problem that’s often connected to both mental and physical illnesses.
In sleep, you are not aware. You do not make decisions. You do not think, feel or experience anything. So in a way, sleep is a little like meditation, except that we lack awareness or control when we’re sleeping. Most of us don’t know how to control our dreams or to wake up on time without an alarm. We also use sleep like a drug, for example if we’re depressed — In the past, I’ve used sleep like a fast forward button to skip the unpleasant parts of life.
So while sleep is necessary (just like thoughts are), sleeping excessively is no more productive than being lost in y our thoughts all the time. Developing a healthy relationship with sleep and all the other vrittis is part of the process of yoga, stilling the mind.
Just a reminder … My main source for these translations of the sutras is The Yoga Sutras of Patanjali translated by Sri Swama Satchidananda. If you’re enjoying reading about the sutras from my perspective, I think you’ll find his commentary on them extraordinary.