The computer in sleep mode eerily mimics the state of sleep in living organisms, the periodic state of physiological rest during which consciousness is suspended and metabolic rate slowed down.
The sleep mode is an in-between state, a digital limbo, suspended between on and off.
This frozen state of suspension is registered on screen as a total absence of light, a deep impenetrable blackness, while the program lies dormant waiting to be reanimated by the touch of a key.
To transition to sleep mode, the computer saves the context of all running programs to memory. It then removes power to the power-hungry Central Processing Unit (CPU), causing it to stop executing instructions. Simultaneously, the display, hard disks, and non-essential peripherals power down. Once this happens, the computer is technically "asleep:"1 the only power the computer draws goes to maintaining the state of its memory and powering the small amount of hardware logic required to wake up. This typically amounts to somewhere between 5—20 Watts, about the same amount of power that an energy-efficient light bulb consumes. When the user wakes the computer, the sleep process reverses: peripherals power up, the CPU loads the context of the user's work from memory, and the user resumes work where she or he left off.
1. The technical term is the Advanced Configuration and Power Interface (ACPI) G1/S3 mode.
Technical description written by Kyle Jamieson, UCL Computer Science