Another possibility is someone in another apartment has one of the
recirculation pumps installed to get hot water to their faucet
quickly. These are small pumps that sit under the sink and when
activated pump water from the hot line into the cold line until actual
hot water arrives, at which point they shut off. Or are supposed to
shut off at least.

I think your thinking of an instahot (sp) witch is no more than a small
water heater.mounted under the sink or lavi. A hot water loop is a loop
that would start and finish at the main hot water tank . Basically it's a
pump that circulates the hot water to each fixture. It would be piped to
every sink feed by the hot water tank. Now there could be something wrong
with a check valve causing some sort of problem . If there is one water
heater all 8 unites then everyone should have the same problem.( could be a
dedicated WH) But what's interesting it happens between 6am and 3am
sometimes the recirculating pumps are controlled by a time clock so they
don't run all the time so I would guess 6am to 3am would be peck demand.