When a computer is in a Sleep state, it is not performing any task and may appear to be off. But it is not shut down but retains the memory state. S0, S1, S2, S3, and S4 are the four power states, of which S1, S2, S3, and S4 are the three sleeping states. With each successive sleep state, from S1 to S4, more of the computer is shut down. S5 is the classic completes shutdown power state.
System Sleep States
In this post, we will see the different System Sleep States in Windows. MSDN explains this quite well.
System Power State S0 – This is the Working State, where your Windows PC is awake. This is not a Sleep state.
System Power State S1 – In this sleep state, the CPU is stopped and your computer is in standby mode. If the next S3 state is note supported, this S2 is the default state on most hardware. The Processor clock is off and bus clocks are stopped. In this state, the power consumption could be between 5 – 30 Watts.
System Power State S2 – This state is similar to S1 except that the CPU context and contents of the system cache are lost because the processor loses power.
System Power State S3 – In this state, data or context is saved to RAM and hard drives, fans, etc. are shut down. The power consumption is usually less than 5 Watts. Wake-On-LAN is supported from S3 (Sleep) or S4 (Hibernate) state in Windows 10/8.
System Power State S4 – In this state, data or context is saved to Disk. It is also known as the Hibernate state and is useful for laptops. Your PC saves the contents of RAM to the hard disk. The hardware powers off all devices. Operating system context, however, is maintained in a hibernate file that the system writes to disk before entering the S4 state. Upon restart, the loader reads this file and jumps to the system’s previous, pre-hibernation location. Power consumption is again less than 5 Watts.
Connected Standby State
In Windows 10/8, there is a new state called the Connected Standby State.
Connected Standby brings the smartphone power model to the PC. It provides an instant on, instant off user experience that users have come to expect on their phone. And just like on the phone, Connected Standby enables the system to stay fresh, up-to-date, and reachable whenever a suitable network is available. Windows 8 supports Connected Standby on low-power PCs platforms that meet specific Windows Certification requirements. In Connected Standby the S3 state is disabled and an additional power state known as S0 Low Power Idle is enabled. Connected Standby systems include Windows RT systems as well as certain other Windows 8 systems.
The Slide To Shut Down feature in Windows 8.1/10 will work only if the hardware supports Connected Standby State.
How does Connected Standby differ from Sleep and Hibernate
Sleep and Hibernate are system-wide coordinated sleep states. When the operating system enters or exits one of these states, it must transition the system in a coordinated manner across applications, services, drivers, devices, and firmware. These transitions require coordination and processing across many layers in the system, many of which are provided by third parties. Therefore, these transitions can be relatively time-consuming and prevent transitions from being near-instant to the user.
Connected Standby is neither a sleep state nor a fully coordinated, system-wide power state transition. In Connected Standby, the system is still on but the display is powered off and the system is driven to be as idle as possible. The goal is to provide a seamless on/off experience and constant connectivity while delivering consistently long battery life. Systems that support Connected Standby do not support Sleep (or ACPI S3) because Connected Standby effectively replaces the Sleep experience. Connected Standby-capable systems running on x86 platforms do support Hibernate. Hibernate is not supported on ARM-based platforms.
This document from Microsoft will tell you more about the Connected Standby State.
Find out if your Windows computer supports Connected Standby State.