Sleep vs Hibernate: What Each Mode Leaves on Disk

Two mechanisms, one goal

Sleep is a pause, not a shutdown. In classic S3 sleep the RAM stays powered and refreshed, everything else — CPU, drives, fans — drops to almost zero, and the session waits to be resumed in about two seconds. Modern standby (S0 low-power idle), the default on laptops for years, keeps the system partially network-active: it wakes for mail, updates and notifications, which is why a laptop in a bag sometimes comes out warm. In neither variant is anything written to disk; your session lives in memory or it lives nowhere.

Hibernate is the opposite trade. Windows compresses the entire RAM image, writes it to hiberfil.sys at the root of C:, and powers the machine off completely — no draw, no warmth, no surprise wake-ups. On resume the file is read back into RAM and you land exactly where you left off, at the price of a boot-like wait measured in tens of seconds. Laptops with modern standby usually drop into hibernate on their own as the battery runs out.

What each mode actually costs you

The practical differences fit into one short list. It is worth checking before you pick a default, because the wrong default is how laptops end up dead in bags and desktops end up with a mystery file eating half the disk.

• Disk: sleep writes nothing; hibernate reserves hiberfil.sys at roughly 40 percent of installed RAM by default, adjustable from 40 to 100 with powercfg /h /size
• Power: classic sleep costs a few percent of battery per day, modern standby more when it keeps waking; hibernate costs nothing once the file is written
• Resume: sleep returns in two to three seconds; hibernate reads the whole memory image back, so 16 GB on a slow SSD takes noticeably longer than 8 GB on a fast one
• Transit: a sleeping laptop in a bag can wake from a network event or a bumped key; hibernate ignores everything until you press the power button
• Wake-on-LAN: a sleeping machine can be woken remotely; hibernate needs firmware support and usually a nearly full boot path anyway

A policy that works

Sleep for breaks and overnight at home; hibernate for bags, planes and the week-long drawer. Reboot about once a week to clear accumulated system state — hibernate does not do that, because it deliberately restores everything you had running. If you never hibernate and do not use fast startup, powercfg /h off deletes hiberfil.sys and hands the space back to C:; that is also the only honest way to reclaim it, since the file cannot be deleted by hand and Windows would recreate it anyway. And one thing worth remembering: hibernate leaves your session on disk in the clear if the drive itself is not encrypted, so on an unencrypted machine with sensitive data, a full shutdown is the cleaner exit.

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