Returning to work after holidays can be stressful. Even if 1st of January felt like any other Sunday, there is a lot built in the "New Year, New You" sentence, even if it is not your mantra.

Setting realistic targets and structure for your day, helps to get back on track. Or set a new one if you decide so.

Part of the healthy structure is getting enough sleep, hence the saying "Sleep on it":
"A good night’s sleep helps your mind process what you encountered during the day so you can efficiently utilize the information later. But it’s also essential for bringing in new information in the first place. 
A University of California, Berkeley study found that getting adequate shut-eye before learning helps recharge our brains and makes them ready to take on new information. 
Russell Sanna, Ph.D., executive director of sleep medicine at Harvard University describes this process by comparing bits of information to pieces of paper, and the brain to a series of filing cabinets. “Different pieces of paper are filed in different portions of the brain, and the pathways to access those different files are consolidated [during] sleep,” he says."

Source: Jared Miller "Does 'Sleeping On It' Really Work?"