This was the study I was thinking about. It’s maybe one reason why animals come out of a state of hibernation to sleep:

Backwards Signals Appear To Sensitize Brain Cells

How nerve cells fire when we’re awake (from dendrite to axon):

During waking hours, electrical signals travel from dendrites – antenna-like projections at one end of the cell – through the cell body. From the cell body, they then travel the length of the axon, a single long projection at the other end of the cell. This electrical signal stimulates the release of chemicals at the end of the axon, which bind to dendrites on adjacent cells, stimulating these recipient cells to fire electrical signals.

How nerve cells fire when we’re asleep (from axon to dendrite):

When the mind is at rest, the electrical signals by which brain cells communicate appear to travel in reverse, wiping out unimportant information in the process, but sensitizing the cells for future sensory learning.

The diagram at the top of this post shows a neuron firing in reverse, when we’re asleep.

Reverse firing clears the brain:

The brain doesn’t store all the information it encounters, so there must be a mechanism for discarding what isn’t important. These reverse brain signals appear to be the mechanism by which the brain clears itself of unimportant information.

But can also reinforce memories:

A connection that is reset but never stimulated again may simply fade from use over time. But when a cell is stimulated again, it fires a stronger signal and may be more easily synchronized to the reinforced signals of other brain cells, all of which act in concert over time.

Sleep is for forgetting.