Why is memory usage increasing even if there is no user in the Rails application on Heroku?

Question

Why is memory usage increasing even if there is no user in the Rails application on Heroku?

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This is the "Memory Usage" of my Rails application on Heroku.

It starts at 290 MB at 2 a.m. and at 12 pm is 412 MB! My web application has something like 5-10users, and I'm sure no one uses it at night. So why is memory usage increasing? How can I check some reasons?

Gemfile

gem 'puma'
gem 'dalli'
gem 'connection_pool'

production.rb

 config.cache_store = :dalli_store,
                    (ENV["MEMCACHIER_SERVERS"] || "").split(","),
                    {:username => ENV["MEMCACHIER_USERNAME"],
                     :password => ENV["MEMCACHIER_PASSWORD"],
                     :failover => true,
                     :socket_timeout => 1.5,
                     :socket_failure_delay => 0.2,
                     :pool_size => 5
                    }