Researchers have found that it may not just be the long hours that leave you feeling as though your in a mental fog at the end of a day in the office - it might be the office itself.
Specifically, the re-circulated air in an office building may be clouding your brain with high levels of carbon dioxide (CO2).
We know that humans can only survive in an environment where there is far more oxygen (which we inhale) than CO2 (a byproduct of our breathing, which we exhale) in the atmosphere.
But recent research has suggested that far lower levels of CO2 than it would take to suffocate our lungs may still be able to starve our brains of oxygen, as the New York Times reported.
That deprivation can make us dumber - and the offices where we're expected to bring our mental best may be the worst environments for brain power.
The EPA considers excessive moisture, volatile organic compounds (VOCs), combustion products, radon, pesticides, dust particles, viruses and bacteria all top air quality problems. And they all exist indoors.
The most clearly tested concern is over CO2, 'but I would imagine that a there is a host of other air pollutants that co-vary with CO2 that are also important to cognitive function,' said Dr William Fisk, a professor at the University of California, Berkeley's Center for the Built Environment.
CO2 is relatively easy to control, so scientists have experimented exposing or observing office workers, school students and even pilots to varying levels of the gas.
The longest-running research has shown that students quite consistently perform more poorly on standardized tests in schools where CO2 levels are higher.
Schools in general tend to have relatively poor ventilation and high CO2 levels, most likely because school budgets are slim, meaning they don't often get updated equipment.
Studies among adults went a step further, however, altering the levels of CO2 in a room where the study participants were asked to complete office work-like tasks that tested their decision making.
Among the first was conducted by Dr Fisk himself (and his team at Berkeley). They had the participants do problem-solving tasks that mimicked the kind of executive brain functions they might need in the workplace, while sitting in rooms that had as little as 600 parts per million (ppm) CO2 and as much as 2,500 ppm.
The more CO2-filled the room, the more poorly the subjects completely the tasks.
Over the course of six days, Harvard University toyed with the CO2 levels in rooms where their study participants did similar kinds of tasks in 2016.
'We saw dramatic differences when they breathed better quality air,' said Dr Joseph Allen, who led that research.
'More fresh air and higher ventilation rates are associated with dramatic benefits for decision-making.'
In other words: 'Better air quality leads to better cognitive functioning among knowledge workers,' Dr Allen said.
High occupancy buildings - like schools and offices - have more air pollution from people and objects to clean out, meaning that their ventilation systems will struggle to keep up if they're not relatively high-efficiency.
Ventilation brings fresh air in while cycling stale air out, so it can be somewhat improved by doing something as simple as opening windows.
But to really optimize a space for workers, companies must invest in good ventilation systems.
The upfront cost might spook employers, but researchers say that the benefit is worth it.
Dr Allen says that work places with better air quality have better records for employee absenteeism, in addition to setting the stage for better, clearer, faster work from those employees.
'Insuring that you're bringing in fresh air from outside and have proper air ventilation and filtering is critically important to commercial office buildings,' he said.
'Not only is it good, when you do, to improve health and productivity [for employees], but it comes with a bottom-line benefit.'