Stamina (also called endurance and energy) is the player's overall energy, represented by a light blue bar on the default HUD, and is used to move. If the player loses too much stamina, the player will be unable to run, jump or even walk. The player slowly loses stamina by walking, if overencumbered, but loses it regardless of conditions by running and jumping. Carrying too much will affect the stamina loss across the board.
In the most basic sense, the player loses stamina by movement, and moving faster will deplete stamina faster, meaning sprinting and jumping will use up stamina while walking will not. Weight also plays a crucial role in stamina loss and gain, as the more weight the player carries, the faster stamina decreases. For example if one is carrying 55 kilograms of gear, above the maximum recommended limit, the player loses stamina by merely walking. Sprinting and jumping with such encumbrance will use up stamina very quickly. If 2/3 of the stamina bar is lost, the player becomes unable to sprint. If it is nearly empty, the player will be unable to move.
The simplest way to restore stamina is by standing still and letting the stamina bar recharge by itself. The fastest way is drinking energy drinks, as one energy drink will restore the bar to 2/3. In Call of Pripyat, the effect is changed to simply increase the regeneration rate drastically. Using endurance restoring artifacts like the Snowflake and Moonlight will also increase the player's stamina gain.