The overall status of how your idols are performing as a unit is indicated by the icon in the circle on the top right.
You will want to get to Enthusiastic and keep it there as much as possible. This becomes fairly easy once you get good at morning and evening communications.
At the start of the game you will be in Normal status. As mood increases the unit status will move towards Enthusiastic and if the mood of any girl decreases the unit status will move towards the various bad statuses depending on their mood balance.
While in Enthusiastic status, an additional point of Unity Score is gained each day.
The Dancing, Sparkling and Humming status appears to be entered depending on which of your unit's Dance, Visual and Vocal statistics is the highest, but it is unclear what triggers it. I have seen it triggered after doing multiple lessons of that type in a row but I have also seen it triggered after doing multiple different lesson types or even multiple stage performances in a row. The actual effects of this status are still unknown and the manual leaves them unclear.
From Normal status, you can get to Fatigued status in as little time as two days if you leave all lessons to your idols and tell them to do them at full power.
If you enter Fatigued status, you lose the opportunity to do evening communications and in addition, if you continue to work them too hard, there is a risk of them not coming to work the next day. Additionally, it is possible for them to skip work without entering Fatigued or Burned Out status before doing so, if they are close to being fatigued due to the previous days. They won't skip work if there is a compulsory event on the next day, though, so if you're trying to work them as hard as possible (e.g. to complete lessons and stages as fast as possible in order to level up and gain money quickly without spending too much real time in the game), this can be exploited.