Considering only 60-80% of our day is actually spent being productive, it makes sense.
When you have less time available you're more likely to focus and less likely to get distracted.
With shorter working hours, you have more time to do the things you enjoy or spend more time with your family and friends.
Even if your not able to work a six-hour work day, set a fixed start and end time to your working day, minimise or reduce your distractions and only spend time on productive tasks. Avoid time wasters, stay focused and you'll at least reduce your working hours.