Psychology Today has this to say about The Difficulties of Multi-tasking:
Multi-tasking, the mental act of juggling, may not actually be the best way to save time or get things done well. A new body of research has found that multi-tasking makes people less efficient and reduces the level of brainpower used for each task. Also, people who overburden their minds with too many tasks at once can have problems with short-term memory.
One study, in the Journal of Experimental Psychology, found that the mind slows down when it switches back and forth between tasks. The only way to turn off this mental friction is to put more time, even just a few seconds, between tasks. A second study, in the journal NeuroImage, also notes that the mind does not cope well with multitasking. It asked participants to listen to sentences while comparing two rotating objects. Even though these tasks use different parts of the brain, visual input dropped 29 percent and listening success fell 53 percent.
For people doing too many things at once, additional worry can build up into a stress response. This adrenalin rush can damage the cells that form new memories. It can also weaken attentiveness and alertness. So what can people to get their act together? Focus on fewer tasks.