**4. Conclusions**

The body of evidence that intends to measure the economic and financial repercussions of the Efficient Market Hypothesis (EMH) on cryptocurrencies has accumulated in an increasing pace. There is already a significant bulk of academic work that provides evidence in favor of ine fficiency in digital currency markets and primarily in the Bitcoin market.

Weak-form e fficiency that shows whether prices reflect the information contained in the past series of prices has been tested in the grea<sup>t</sup> majority of the thirty-eight studies under scrutiny. The largest number of academic papers examined advocate that the EMH does not hold. Long memory in cryptocurrency time series is detected as dependence from past returns is revealed. A battery of tests

from a wide spectrum are employed such as the Hurst exponent, the Bartels test, the variance ratio test and its specifications among others. Thereby, investors can use past information to predict future returns. This enables speculators to exercise profitable trading strategies suffering only very low risk.

It should be noted that long-range dependence that leads to inefficiency is found to fade out as time passes in the Bitcoin markets as well as in the cryptocurrencies market in general. This provides useful feedback and generates an even more vivid debate about the future of digital currencies. These coins constitute extremely sophisticated investment assets that have attracted an overwhelmingly upcoming number of investors and are expected to become a cornerstone in finance.

The present study fills a gap in relevant literature by providing an overall perspective of the efficiency characteristics and profit opportunities in digital currency markets. Therefore, this systematic survey enlightens policymakers, academics, investors and the economic press about the profitability dynamics inhibited in the markets of these innovative liquidity forms. Avenues for future research include the mapping of the efficiency-measuring dynamics of every methodology adopted in the relevant literature. Moreover, the connection of investor sentiment with the speed of adjustment of cryptocurrency markets towards validity of the Efficient Market Hypothesis should pave the way for new empirical examinations and systematic surveys.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.
