You can go to settings - battery - Options, and select wake for network access to Always. It will update your iCloud and iMessage etc more often when not plugged into power, but when the machine is in sleep, network won't work the same way as when it is on, and if this happens when you change location i.e. Home & school that is normal and might just be because other people are using the wifi. MacBook/pc sleep is not the same as when the Phone is in sleep.