Nursing your little one reminds you to sit down, put your feet up and take regular breaks. Allow yourself to get the rest you need during the day. Lie down at least twice, and bring your baby to bed with you. You didn't mention the age of your older child, but most likely, if you childproof your bedroom, he can either nap with you and baby, or play quietly while you rest. I would not recommend feeding your baby formula at night to encourage him to sleep more deeply.
Babies sleep differently than adults.
Their short sleep cycles and more frequent periods of arousal very likely exist for a good reason. This type of sleep cycle may be crucial to their survival. Sharing sleep may prevent long episodes of deep sleep. Though the cause of SIDS is still not known, many experts believe that it involves the inability of the brain to wake the sleeping baby if breathing stops. Sharing sleep also encourages breastfeeding, which is thought to be protective against SIDS, and regulation of the baby's respiration and heart rate.