Firing on all cylinders all of the time is not healthy. Giving 110% is not sustainable, even during times of rapid growth - which is also not sustainable. Maximum capacity is not a good thing for your psyche, just like 0% unemployment would not be a good thing for our economy. In my opinion, sometimes it's quite alright to be completely unproductive (of course one needs to earn that right first).
There's a popular statistic in the field of organizational behavior that basically says employees can get away with working at only 60% productivity and still keep their jobs. Therefore, through correct situational leadership practices a manager could potentially influence productivity by 30-35% (maximum productivity is really 95%, 110% is a myth).
If you happen to be a manager of people, it's important to realize that your coworkers and subordinates are also human...just like you! They will get burned out from time to time and their motivation, productivity, and creativity will lag just like yours does. This is indeed very frustrating for fresh managers because they often think if they tamper with this and fiddle with that, then maybe, just maybe, they'll be able to extract just a little bit more productivity from their workforce. This works, but it's counterfeit. You can only get away with insincere motivate for a short time before people start resenting you.
Don't quarrel with reality. I would venture to say that it is practically impossible to battle with the innate rhythm of productivity and motivation. It's not Ok to sit back and do nothing, although that is what most managers do by conveniently becoming busy with something else in hopes to avoid confrontation.
If you discover that a great employee has become disenchanted with their job, then talk to them about it. Get the issue out in the open. Maybe they just need a reassignment of duties, or maybe the issue stems from their personal life. If you can help, then help. If it's beyond your control, let the employee work out the problem on their own. The solution could be as simple as a sabbatical.
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