Sleep is not really Nibbana because Nibbana is defined as the "uprooting" & "destruction" of the mental defilments, as follows:
To whatever extent there are phenomena conditioned or unconditioned, dispassion is declared the foremost among them, that is, the crushing
of pride, the removal of thirst, the uprooting of attachment, the
termination of the round, the destruction of craving, dispassion,
In sleep, the mental defilements of an non-fully-enlightened mind remain dormant as "underlying tendencies". Therefore, sleep cannot be Nibbana.
However, the teacher Bhikkhu Buddhadasa make some comments about how the 'Nibbana element' operates in everyday life, here: Nibbana for Everyone. which includes 'sleep'.
Nibbāna is one of the dhātus (natural elements). It is the coolness that remains when the defilements – greed, anger, fear, delusion –
Any reactive emotion that arises ceases when its causes and conditions cease. Although it may be a temporary quenching,
merely a temporary coolness, it is still Nibbāna, even if only
temporarily. Thus, there’s a temporary Nibbāna for those who can’t yet
avoid some defilements. It is this temporary Nibbāna that sustains the
lives of beings who continue hanging onto defilement. Anyone can see
that if the egoistic emotions existed night and day without any pause
or rest, no life could endure it. If such life didn’t die, it would go
crazy and then die in the end. You ought to consider carefully the
fact that life can survive only because there are periods when the
defilements don’t roast it. These periods outnumber the times when the
These periodic Nibbānas sustain life for all of us, without excepting even animals, which have their levels of Nibbāna, too. We are able to
survive because this kind of Nibbāna nurtures us, until it becomes the
most ordinary habit of life and of mind. Whenever there is freedom
from defilement, then there is the value and meaning of Nibbāna. This
must occur fairly often for living things to survive. That we have
some time to relax both bodily and mentally provides us with the
freshness and vitality needed to live.