The present presses automatically on you. The future does not. To attend to the future requires bandwidth, which scarcity taxes. When scarcity taxes our bandwidth, we become even more focused on the here and now. We need cognitive resources to gauge future needs, and we need executive control to resist present temptations. As it taxes our bandwidth, scarcity focuses on the present, and leads us to borrow.
Some struggle with medical issues - like insomnia - that make sleep hard. But for many of us, the quantity and quality of sleep come down to a matter of choice. Still, only a few enterprising economists have looked closely at this, and generally, those have assumed that we choose our hours of sleep optimally.