Not really. It is important to maintain conscious. When you simply "focus on just that" you may then just wake up once the task is complete and find out you've went through it automatically like if it was a [non-lucid] dream.
As about meditating while sleeping - I have not explored this part enough. Renowned Dzogchen teachers Tenzin Wangyal Rinpoche and Chogyal Namkhai Norbu Rinpoche have books on this subject. A Zen teacher quoted by Alan Watts also said "when you are hungry - eat, when you are tired - sleep" and explained that ordinary people don't actually follow this as they think about countless things while eating and see countless dreams while sleeping. If we go slightly farther than secular mindfulness dares to go and consider the Buddhist religious goal of after-death liberation, they say "dream yoga is not an essential part of practice, but how do you expect to stay conscious in death if you can't even stay conscious in sleeping"?
As for me, I consider unmindfullness a voluntary death, throwing parts of the life time you have been given (by the G-d, or figuratively - whatever you prefer to believe, this doesn't change the matter) away. You only live and actually own/use your time when mindful. When you are not it just disappears. AFAIK even Immanuel Kant believed something like this.