The augmentation, Circadian Regulation (Core Rulebook, p. 304), is the augmentation that allows biomorphs to live and thrive on 2 hours of sleep every 24 hours, or 4 hours every 48 hours. The augmentation allows a biomorph to wake up or fall asleep almost instantly.

The question of infomorph sleep is elaborated in 'Transhuman'.

Quote:

For infomorphs that regularly deal with biologicals, a cycle of activity divided into three watches tends to emerge. Watches are of 8–10 hours in length, depending upon whether the local day cycle is 24 or 30 hours. The first watch, corresponding to daytime (or whatever period of time the majority of the biologicals are active and working) is spent on necessary work tasks. The second watch may be spent on socializing, recreation, or hobbies. The third watch is devoted to self-maintenance, reflection on the prior day’s events, and preparation for the next day—which often includes getting a head start on work.

It also mentions that while infomorphs are basically tireless, they still do get *bored* with their work, hence the three-cycle day including self-maintenance/introspective downtime. Since an ego in a synth shell is basically an infomorph run on a cyberbrain instead of an emulated cyberbrain, I'd say that similar rules apply to synths. Additionally, both synths (and infomorphs, I guess) may use legal narcoalgorithms to induce a dream-like state while nanobots do some maintenance work in their sleeping pod, which are apparently a thing, as described in Transhuman:

Quote:

Synthmorph:You actually have a small private storage pod for downtime and to keep a few possessions.

Sleeping doesn't only let the brain have downtime; it allows your entire body to flush built-up toxins, replenish your immune system, and heal without the body doing annoying things like run around. Pods would need this. Presumably they have basic biomods as standard, which I guess is what reduces that down to 4 hours. They should be able to have Circadian Regulation installed into a pod, like most bioware. Of course, Medichines can do this too, but for game mechanics sake you could argue that you have to filter dead nanomachines from your bloodstream sometime. The cyberbrain in the pod could stay active during that time, though. Presumably egos use the time to run their rapid-sleep-emulation that nerdnumber1 suggested, or run the latest MRGCNN XP. The rapid-sleep-emulation could do what we currently think sleep does. Neural growth and pruning, allowing the formation of long-term memories and integration of new skills. In infomorphs this basically means reformatting your recent memories and integrating it with the ego, a one-ego merge, if you will.