It's important to our mental and physical health . From a quick cat nap to long , deep beauty sleep , we have many expressions to describe sleep . Of course , cats sleep for many hours during the day . That's why we call a short period of sleep – preferably in a warm spot of the room – a cat nap . The purpose of a cat nap is different from that of a power nap . We take power naps during the middle of a workday to help us get more work done . If you had a really tough week at work , you might want to sleep in on the weekend . This means you sleep later than usual . Teenagers are especially known for sleeping in when they don't have school . It's not hard to understand where we get some of our sleep expressions . For example , when we're tired , we may say we need to get some shut-eye . You can only sleep when you shut your eyes . Although , I have heard stories about people who sleep with their eyes open – which can look kind of creepy . When we use this expression , please note that it is singular . If you are feeling tired , you might need to catch some z's . “Z ” is the sound we use to represent sleeping . So , to catch some z's simply means to sleep . Now , sometimes we fall asleep when we don't mean to . We might nod off while sitting somewhere and waiting , like in a doctor's office or on the train after a long day of work . A “nod ” is the up and down movement of the head . That head movement is what happens when you nod off while sitting . Your head drops down and … you fall … asleep . Now , we might take a short snooze during the day to get a little rest . This lets us get a little more shut-eye . Most of these expressions are for the short rests we take during the day -- but not when we actually go to bed at night . We have a whole different set of other expressions just for that ! You can simply turn in . This just means to go to bed for the night . When you fold down the blankets to get into bed , you are “turning down the bed .” Some people are just too tired to fuss with turning down blankets . They might say , “It's late . I'm going to crash .” This means they are so tired that they feel like they need to fall -- or crash -- onto the bed . Young people are fond of this expression . If you hit the sack or hit the hay you are officially going to bed for the night . A sack is a big bag , like the bag that rice comes in . Word historians say that in earlier days , people would fill empty rice sacks with something soft , like feathers , and make it a pillow . And hay is the kind of dried grass that farmers put down for some farm animals to sleep on . It's actually quite comfortable . In fact , a long time ago , some beds were filled with hay . Maybe you are one of those lucky people who drift off to sleep quickly . As soon as your head hits the pillow , you are out like a light . This means you fell asleep in the time it takes to turn off the light . For some other people , sleep might not come so fast . Some people might need to listen to peaceful music or read a book to help them fall asleep . Others might need to count sheep . When you count sheep , you imagine cute little sheep jumping over a fence . The scene is so relaxing -- and counting sheep is so boring -- that will you soon be sawing logs . That is to say , you are so deeply asleep that you will start snoring loudly . Some people may fall asleep quickly , but then they have a restless night's sleep . They toss and turn all night . And what about those times when you can't seem to fall sleep at all ? For those nights you can say , “I didn't sleep a wink !” Now , a couple of expressions with the word “sleep ” aren't actually about sleep at all . Sometimes when we have something on our mind that is important to us , we might think about it as we're falling asleep . It might keep us awake . We often use this to answer something said to us . Here's an example : Curtis is really mad at you . He said he's not going to forget what you did at the party last night . I didn't do anything . I'm not going to lose any sleep over his empty threats . Now , if you want someone to spend more time thinking about something , you can tell them to sleep on it . We usually say this when someone has to make a decision . Let's hear how to use this expression . In this short conversation , one person wants to get away for some R&R , also known as “rest and relaxation .” The other doesn't feel so free . Okay , so here's the plan . If we leave straight from work on Friday and drive to the cabin , we should get there by 11 . That gives us all day Saturday , Sunday and Monday to ski or hike in the mountains . We can head home Monday night . I'm not sure if I can miss work on Monday . But it's a holiday . But I also need to work on a big project . I think I'll actually feel more rested and relaxed if I work on it . Look , you don't have to make up your mind right now . Sleep on it and let me know by Thursday if you can go . And as we like to say to our children : “Sleep tight .