Hi, Jeremiah. There is no one answer to your question because it depends on the culture of the game company, the attitude of management, the type of game project, and so on. And there are many different types of designer jobs. Unless you're working on your own solo game or for a tiny company, there can be any number of designers performing different tasks on a game, such as a level designer, systems designer, feature designer, and so on.
As Mark said (in a previous answer), game design isn't physically demanding. In fact, it's the opposite --you spend too much time sitting at a computer, which can be unhealthy in itself. I personally know of one person at a game company who was diagnosed with Vitamin D deficiency from never getting outside in the sun. Game designers have to learn to lead a healthy lifestyle, but many don't.
Mental work is still work, though. It burns up energy and can make you tired and stressed if you don't practice the routines of a healthy lifestyle. Ultimately, it's up to you to make sure you get enough sleep, the right kind of food, and frequent exercise, regardless of what job you do.
There are some companies that have poor management and will run you into the ground. It's called "crunch" or "crunching". It's an old practice in the game industry, and a bad one that needs to be stamped out. A well-run company with good management will schedule a game properly to avoid crunch time. A badly-run company can be notorious for crunch time. Some less savory companies love to hire young, eager, new designers straight out of school that are so desperate to work in game design, they'll work any kind of hours, to the point of abuse. Beware of companies that have crunch time.
Doing research into various game companies might help you to avoid crunch time should you go for a career as a game designer. I worked at Zynga and never once had to worry about crunch time. Zynga made casual games, which is an entirely different type of company from one that makes AAA console games. The size and make-up of the teams is different, the scope of the projects is different, the timeline is different -- that's why I wrote up front that there's no one answer to this.
This professional recommends the following next steps:
- Go to gamasutra.com and read articles about the different types of game design. Figure out what type of game design appeals to you.
- There are schools that specialize in game design. Investigate those schools to learn more about the different types of game design. Three prominent schools are Digipen, SMU Guildhall, and Full Sail University.