Mentally, poor sleep worsens your mood, lowers your pain threshold, and interferes with learning and memory. It also diminishes your ability to concentrate and makes you more impulsive. Physically, it increases blood pressure, elevates stress, and harms the immune system. It can even cause weight gain. Poor sleep also has numerous negative effects on the brain, particularly in the prefrontal cortex and hippocampus. It also alters the function of the serotonin, dopamine, and norepinephrine systems.