system
Given the following post, respond as if you're commenting on Reddit:
human
What's the difference between actual sleep and just laying down when fighting off a disease?

I've currently got a cold, so my body needs rest to fight it off and recover.

What's the biological difference between having an actual sleep, or just laying horizontal and watching tv/reading ?

Is my immune system doing the same thing?

Do I actually need more sleep when sick, or does physical inactivity suffice?
gpt
Your body is doing different things when sleeping and when just laying down, resting.  
While sleeping most systems of the body go in a less active mode. The brain lowers its activity and does mainly "maintenance" work, organizing/sorting and "cleaning" stuff. Heart rate goes down, breathing slows, digestive system goes into a less active mode, body temperature drops, hormonal activity changes etc. Immune system is more effective with fighting disease during sleep as some agents that fight disease are higher in levels during sleep. Also sleep deprivation might lower white cell count - the "warriors" that fight disease. Also, when your body relaxes and is not strained it has more resources to fight.  
And we reach to the next part - just laying down, relaxing. While not as effective as sleep, for fighting disease it's still more beneficial than normal activity, because less resources are used for activity that is not connected directly with fighting disease, hence the body can "schedule" more resource for fighting the disease.  
Since a human, unless extreme situations, can not sleep for very long periods of time without waking up, when you are ill, it's still better, to lay down and relax, while awaken, than doing usual activity (which would be hard anyway). The need to lay down and the feeling of being exhausted is a sign, the body is trying to divert resource elsewhere and it's losing energy, that would otherwise be used for normal activity. Putting as less stress on the system as possible, while it's fighting with an external agent is beneficial, hence "keep the bed" advice :).