First of all, there is nothing like a safe investment when it comes to Bitcoin investment but it is a fact that holding Bitcoin for a long term can be much profitable compared to short-term investments but like I said initially there's nothing like a safe investment.
Either you are holding for longtime, or you are holding for a short period of time, the two is risky, but long time investment is less risky compare to short time investment, thats why I do encourage people to hold for longtime.
The main reason why a lot of people lose when investing in Bitcoin is when they sell their coins due to panic and this affects Short term holders more than those that invest for a long term. Bitcoin isn't a fast way to make money, a lot of people start investing with the mentality of getting rich within a short period of time but it doesn't exactly work that way.
If you are planning to make money from bitcoin investment, then you will have to hold your bitcoin for a long term, if you invest in bitcoin hoping to make quick money, you might just end up regretting at the end, because you might be losing instead of making profits, thats why its always encourage to invest with the money which we wont be using at that moment.
Bitcoin investment is for the strong minded people, if you want to hold, even if bitcoin price is dropping, their is nothing to panic about, you just have to keep on holding, and if you having extra funds, you can buy more bitcoin at that moment.