That's a GSM band. If there's still GSM service in that band, it's very common to see Mobile Network Operators (MNOs) turn of base stations at night, effectively enlarging the remaining cells. I've witnessed that very drastically in a German city center, where telefonica used to completely switch of one of their 3G base stations.
This is done because power consumption is the nr. 1 cost driver in established cellular networks for MNOs.
850 MHz is by now also used as LTE/4G band by sprint. LTE is even more flexible in all aspects of power management.
In any case, you can probably simply observe the amount of power the base station observes as uplink from the cellular phones – more phones doing data or voice means more power in the air. It's really that simple. Every bit you transport needs fractions of a Joule. Transport more bits, see more power.
Downlink is just the same; when there's fewer phones that get data, you can more generously assign spectrum to each user, meaning that you need less power density to service these users with the same level of service. In a deep-night scenario, it's thoroughly possible that a vast amount of the time/frequency plane is simply unused. You have to buy your spectrum allocations for the peak volume you want to service. That means you underutilize it when there's noone to request that.