"It's something a lot of people don't talk about," Applegate, 45, tells PEOPLE. "Millions and millions of people aren't sleeping… It affects your spiritual self, emotional self and physical self."
The actress, who averages only three hours of sleep a night, says her insomnia – which she developed around 20 years ago – changed when she became a mother to 5-year-old Sadie Grace in 2011.
"In my 20s and 30s, I used to never be able to fall asleep and would just stay up all night long," the star of the upcoming comedy Bad Moms says. "But when my daughter was born, I got into a three-hour feeding cycle. Now she sleeps 10 hours a night, but my body still wakes up every few hours. I'll also be up from 2 a.m. to 6 a.m. and finally fall asleep. And then [Sadie Grace] comes in and is ready for breakfast!"
Why So Awake, an online resource for insomniacs, cites a person's behavior and neurotransmitter activity in the sleep and wake systems as the main causes of the disorder.
To help ensure a good night's sleep, Applegate follows a handful of personal rules she has laid out for herself.
1. Keep it Cold
"It should be about 60 to 68 degrees, it should never be any warmer than that," says Applegate. "You've got covers, if you get cold, put some socks on!"
2. Blue Light OFF
"There's something in the blue light from the smart phone and from the television that stimulate your awake brain, which at night time is supposed to be the asleep brain. You should not have blue light on an hour before sleep, which I know is really hard for people because we check our phones constantly," explains the actress.
3. Use a straw!
The mother of one says that, for her, a no-slurping policy is a must.
"There are so many factors, habitual things, that contribute to not sleeping. If you have water next to your bed and you have a partner, make sure to have straws, because they cut down the noise. My husband would have water in the middle of the night and that would wake me up with that gulping sound," Applegate says of hubby Martyn LeNoble.
4. Sleep Timer is Your Friend
"If you have to fall asleep to television, make sure you set your timer for it to shut off. It's not good for your subconscious to listen to the nonsense on late-night news," she says.
5. Sorry, No Pets Allowed
Applegate admits this rule isn't her favorite.
"We have pets – we love our pets – but we had to start not having them in the bedroom because we couldn't sleep," she says. "They would constantly roll around and move. It was the saddest thing."