How does a computer sleep?
When you close a laptop, the chip enters sleep: most of it turns off, while just enough stays on to wake up quickly.
1. Most parts switch off
When a laptop sleeps, the screen, Wi-Fi and most of the chip switch off. It is like a house at night: lights off, fridge on.
2. Memory keeps your work
Memory holds your open apps, not saved files, and keeps them alive by itself (self-refresh), using a tiny bit of power. Unlike food in a fridge, this data is lost within moments if the power stops.
3. Deeper rest saves more power
A computer's power levels are named S0 (on) to S5 (off); S1 and S2 are rarely used today. Deeper levels save more power but take longer to wake.
4. Modern standby: on, but napping
Newer laptops often use modern standby, napping like a phone with its screen off. The system stays on (S0), but the chip rests at very low power (S0ix) and sometimes wakes for a moment.
5. One busy part drains the battery
One busy part, or its driver software, can keep the whole chip awake. It is like a doorbell ringing all night, and the battery drains while the laptop seems asleep.
Words to know
- Sleep (S3)
- Rest with memory still powered. It wakes in seconds.
- Hibernate (S4)
- Memory is copied to storage (the disk), then the power goes off.
- Modern standby
- The system stays on (S0) while the chip rests in very low-power states called S0ix.
- Self-refresh
- Memory keeping its data alive by itself while the chip rests.
- Driver
- Software that lets the operating system use one part, such as Wi-Fi.
Where you meet it at work
Testers run many sleep and wake cycles, checking that each reaches low power and that everything works after waking. The operating system's sleep report shows the time spent in low power.
Check yourself
A laptop slept all night but lost half its battery. What is a likely cause?Think first, then tap to see the answer.
A busy part, or its driver, kept the chip from resting.