For laptops with full disk encryption or home folder encryption, one of the risks if it is stolen while in sleep mode is that the encryption key is stored in memory and can be read if an attacker knows how. To me, it seems that, in theory, operating systems should be able to have a "secure sleep" option where the key is erased from memory prior to sleep mode, and on resume, the user must provide a password to unlock the encryption key, as through a cold boot. All processes except the lock screen would be prevented from continuing until the encryption key is restored to memory.
I realize that this would mean that the computer cannot do any scheduled tasks in sleep mode, but most users probably wouldn't care about that. And maybe drivers or other obstacles would prevent this from being realistic.
Are there any reasons why a "secure sleep" option cannot be easily implemented?