Let me begin by saying we are not outside for 4-6 hours everyday. We aim for 4-6 hours outside at least three to four times a week (a little more in the nicer months and a little less in the worse ones). I first came across this time recommendation in the works of Charlotte Mason, a classical English educator who lived at the turn of the 20th century.
I originally thought her idea was utterly preposterous. It is a far cry from typical child activities these days, most of which last no longer than an hour. At the invite of a friend we began to spend large immersive chunks of time in nature and immediately my eyes were opened. Children who are allowed this freedom of time outside get lost in nature. They get lost in their imaginations and they get lost in wonder. And then they rapidly develop. There are many factors why but one reason is due to the rich sensory environment that nature always provides.
Our 1000 Hours Outside Goal is loosely based on the writings of Charlotte Mason but we have adapted to make room for daily life. Interestingly, her findings about what is healthy for development of the whole child are consistently being backed by research over 100 years later.