Acer Aspire S5-391 User Manual page 51

Deep Sleep and Sleep mode work together to ensure that your computer
saves as much power as possible while still enabling quick, instant on
access.
To save more power, your computer will typically enter and stay in Deep
Sleep during times when it will not be frequently accessed, for example,
during the middle of the night. To provide quicker access, your computer
will enter and stay in Sleep mode during times when it will be accessed
more frequently, for example, during working hours.
Power management - 51